The North American Numbering Plan (NANP) is a telephone numbering plan that encompasses 25 distinct regions in twenty countries primarily in North America, including the Caribbean and the U.S. territories. Mexico withdrew from the NANP about 1990, although before that area code 905 was used for Mexico City and 903 for Tijuana, whose long distance service was via the United States. Cuba and Haiti have never been part of the NANP.
The NANP was originally devised in the 1940s by AT&T for the Bell System and independent telephone operators in North America, to unify the diverse local numbering plans that had been established in the preceding decades. AT&T continued to administer the numbering plan until the breakup of the Bell System when administration was delegated to the North American Numbering Plan Administration (NANPA), a service that has been procured from the private sector by the Federal Communications Commission (FCC) in the United States. Each participating country forms a regulatory authority that has plenary control over local numbering resources. The FCC also serves as the U.S. regulator. Canadian numbering decisions are made by the Canadian Numbering Administration Consortium.
The NANP divides the territories of its members into numbering plan areas (NPAs) which are encoded numerically with a three-digit telephone number prefix, commonly called the area code. Each telephone is assigned a seven-digit telephone number unique only within its respective plan area. The telephone number consists of a three-digit central office code and a four-digit station number. The combination of an area code and the telephone number serves as a destination routing address in the public switched telephone network (PSTN). For international call routing, the NANP has been assigned the international calling code 1 by the International Telecommunications Union (ITU). The North American Numbering Plan conforms with ITU Recommendation E.164, which establishes an international numbering framework.
From its beginnings in 1876 and throughout the first part of the 20th century, the Bell System grew from essentially local or regional telephone systems. These systems expanded by growing their subscriber bases, as well as increasing their service areas by implementing additional local exchanges that were interconnected with tie trunks. It was the responsibility of each local administration to design telephone numbering plans that accommodated the local requirements and growth. As a result, the Bell System as a whole developed into an unorganized system of many differing local numbering systems. The diversity impeded the efficient operation and interconnection of exchanges into a nationwide system for long-distance telephone communication. By the 1940s, the Bell System set out to unify the various numbering plans in existence and developed the North American Numbering Plan as a unified, systematic approach to efficient long-distance service that eventually did not require the involvement of switchboard operators.
The new numbering plan was officially accepted in October 1947, dividing most of North America into 86 Numbering Plan Areas (NPAs). Each NPA was assigned a Numbering Plan Area code, often abbreviated as area code. These codes were first used by long-distance operators to establish long-distance calls between toll offices. The first customer-dialed direct call using area codes was made on November 10, 1951, from Englewood, New Jersey, to Alameda, California.Direct distance dialing (DDD) was subsequently introduced across the country and by the early 1960s most areas of the Bell System had been converted and it was commonplace in cities and most larger towns.
In the following decades, the system expanded to include all of the United States and its territories, Canada, Bermuda, and 17 nations of the Caribbean. By 1967, 129 area codes had been assigned.
At the request of the British Colonial Office, the numbering plan was first expanded to Bermuda and the British West Indies because of their historic telecommunications administration through Canada as parts of the British Empire, and their continued associations with Canada, especially during the years of the telegraph and the All Red Line system.
Not all North American countries participate in the NANP. Exceptions include Mexico, Greenland, Saint Pierre and Miquelon, the Central American countries and some Caribbean countries (Cuba, Haiti, and the French Caribbean). The only Spanish-speaking state in the system is the Dominican Republic. Mexican participation was planned, but implementation stopped after three area codes (706, 903 and 905) had been assigned and Mexico opted for an international numbering format, using country code 52. The area codes in use were withdrawn in 1991 by the NANPA. Area code 905, formerly for Mexico City, was reassigned to a split of area code 416 in the Greater Toronto Area; area code 706, which had formerly served Mexico's Baja Peninsula, was reassigned to a portion of northern Georgia surrounding the Atlanta region, which retained 404; and area code 903, which served a small portion of northern Mexico, was reassigned to northeastern Texas when it split from area code 214.
The NANP is administered by the North American Numbering Plan Administration (NANPA). Today, this function is overseen by the Federal Communications Commission, which assumed the responsibility upon the breakup of the Bell System. The FCC solicits private sector contracts for the role of the administrator. Initially, the service was provided by a division of Lockheed Martin. In 1997, the contract was awarded to Neustar Inc. which was spun off from Lockheed for this purpose. In 2012, the contract was renewed until 2017. In 2015 the contract from 2017 was granted to Ericsson.
The vision and goal of the architects of the North American Numbering Plan was a system by which telephone subscribers in the United States and Canada could themselves dial and establish a telephone call to any other subscriber without the assistance of switchboard operators. While this required an expansion of most existing local numbering plans, many of which required only four or five digits to be dialed, or even fewer in small communities, the plan was designed to enable local telephone companies to make as few changes as possible in their systems. Most subscribers should not have to dial a long, full national telephone number to make a local telephone call.
The new numbering plan divided the North American continent into regional service areas, called Numbering Plan Areas (NPA), primarily based on the jurisdictional boundaries of U.S. states and Canadian provinces. Each NPA was identified by a three-digit code number. The Numbering Plan Areas were created in accordance with principles that were deemed to maximize customer understanding and minimize the dialing effort, while reducing plant cost.
The existing telephone exchanges and central offices became the local exchange points in the nationwide system, and each was assigned a number of three digits, which was unique in the Numbering Plan Area, but of similar form to all others. These codes served as destination routing codes for use by operators and subscribers to reach any central office through the switching network.
The most populous states required division into multiple Numbering Plan Areas, as each NPA was limited to 540 unique central office codes. For example, New York state was initially divided into five areas, the most of any state, followed by Illinois, Ohio, Pennsylvania and Texas with four NPAs each. These divisions attempted to avoid cutting across heavy toll traffic routes, so that most toll traffic remained within an area, and outgoing traffic in one area would not be tributary to toll offices in an adjacent area.
Traditionally, each central office could serve up to 10,000 subscriber numbers, resulting in a four-digit line or station number. Thus, the new numbering plan identified each telephone in the system with the combination of the area code, the central office code, and the line number, resulting in a closed telephone numbering plan with a ten-digit national telephone number for each telephone. The leading part of this address was the area code (three digits), followed by the seven-digit subscriber number consisting of three digits for the central office and four digits for the station. The intent was that subscribers should not have to dial an area code when making a local call or a call within their plan area, resulting in seven-digit dialing. Area codes were only required in ten-digit dialing when placing foreign area calls to subscribers in another state or numbering plan area.
The new network design, completed in 1947, provided for 152 area codes, each with a capacity to serve 540 central offices. Originally only 86 area codes were assigned. New Jersey received the first area code in the new system, area code 201. The second area code, 202 was assigned to the District of Columbia. The allocation of area codes was readjusted as early as 1948 to account for inadequacies in some metropolitan areas. For example, Indiana area code 317 was split to provide a larger number pool in the Indiana suburbs of Chicago (area code 219).
Initially, states that were divided into multiple areas were assigned area codes with a 1 in the middle, the second position, while areas that covered an entire state or province, were numbered with the digit 0 as the middle digit. However, this rule was removed by the early 1950s. In order to distinguish seven-digit dialing from ten-digit dialing, central office codes were restricted to not having a 0 or 1 in the middle position. This was already common practice, because the system of using the initial letters of central office names did not assign letters to digits 1 and 0. Furthermore, area codes and central office codes could not start with 0 or 1, because 0 was used for operator assistance, and a leading single pulse, i.e. the digit 1, was automatically ignored by most switching equipment of the time. In addition, the eight codes of the form N11 (N = 2-9) were reserved as service codes. The easily recognizable codes of the form N00 were available in the numbering plan, but were not initially included in assignments.
Later, additional area code patterns were assigned for other services, for example, the area codes N10 were used for a period of time for the Teletypewriter Exchange Service (TWX).
The central office code was chosen such that it could be represented by the first two letters of the central office name according to a digit-to-letter mapping that was printed on the face of a rotary dial, by grouping a set of letters with the digits 2 through 9. Such letter translations, designed by W.G. Blauvelt in 1917, had been used in the Bell System in large metropolitan areas since the late 1910s. The network reorganization eventually resulted in a 2-letter 5-digit (2L-5N) representation of telephone numbers for every exchange in North America.
The original plan of 1947 had been projected to be usable beyond the year 2000. However, by the late 1950s it became apparent that it would be outgrown by about 1975.
In the late 1980s and early 1990s, the NANPA implemented calling procedures that required all long-distance calls within an area code to be prefixed with the area code, in an effort to make it possible to assign central-office prefixes with 0 or 1 in the middle position (except for N11), which would otherwise be wrongly taken by the local telephone switch to be the area code. As it had nearly run out of area codes using the existing assignment methods, this expanded the number pool for each area code by nearly 25%, and also allowed for the later addition of area codes with middle digits other than 0 or 1.
Requiring a 1 to be dialed before the full number in some areas also provided for area codes of the form N10, such as 210 in the San Antonio, Texas, area and 410 in eastern Maryland. Therefore, someone calling from San Jose, California, to Los Angeles before the change would have dialed 213-555-0123 and after the change 1-213-555-0123, which permitted the use of 213 as an exchange prefix in the San Jose area. The preceding 1 also ideally indicates a toll call, however this is inconsistent across the NANP because the FCC has left it to the U.S. state public utilities commissions to regulate for traditional landlines, and it has since become moot for mobile phones and digital VoIP services which now offer nationwide calling without the extra digit.
The NANP number format may be summarized in the notation NPA-NXX-xxxx:
|NPA||Numbering plan area code||Allowed ranges: [2-9] for the first digit, and [0-9] for the second and third digits. When the second and third digits of an area code are the same, that code is called an easily recognizable code (ERC). ERCs designate special services; e.g., 888 for toll-free service. The NANP is not assigning area codes with 9 as the second digit.||Covers Canada, the United States, parts of the Caribbean Sea, and some Atlantic and Pacific islands. The area code is often enclosed in parentheses.|
|NXX||Central office (exchange) code||Allowed ranges: [2-9] for the first digit, and [0-9] for both the second and third digits (however, in geographic area codes the third digit of the exchange cannot be 1 if the second digit is also 1).||Central office code: Within the NPA a uniquely assigned three-digit code. It is often considered a part of the subscriber number.|
|xxxx||Line number or subscriber number||[0-9] for each of the four digits.||Within a central office, a unique four-digit number, also called station code.|
For example, 234-235-5678 is a valid telephone number with area code 234, central office prefix (exchange) 235, and line number 5678. The number 234-911-5678 is invalid, because the central office code must not be in the form N11. 314-159-2653 is invalid, because the office code must not begin with 1. 123-234-5678 is invalid, because the NPA must not begin with 0 or 1.
The country calling code for all countries participating in the NANP is 1. In international format, an NANP number should be listed as +13015550100, where 301 is an area code (Maryland).
Each three-digit area code has a capacity of 7,919,900 telephone numbers:
Various office codes in certain plan areas are deliberately not issued, for example, numbers 212718-xxxx, where 212 and 718 are both New York City area codes, are typically avoided to prevent confusion between an area code and a similarly numbered local exchange in the same region. 958-xxxx and 959-xxxx are usually test numbers. Using 0 or 1 as the first digit of an area code or seven-digit local number is invalid, as is a 9 as the middle digit of an area code; these are trunk prefixes or reserved for North American Numbering Plan expansion. Lists of exchanges in an individual area code (posted by CNAC in Canada, NANP in the United States) all list various prefixes as deliberately not issued.
While the national numbering plan of the NANP was designed as a 10-digit closed plan, international direct distance dialing (IDDD) was accomplished by extensive modifications in switching systems to accommodate an open international numbering plan for telephone numbers from 7 to 12 digits.
Canada and the United States have experienced rapid growth in the number of area codes, particularly between 1990 and 2005. The widespread adoption of fax, modem, and mobile phone communication, as well as the deregulation of local telecommunication services in the United States in the mid-1990s, increased the demand for telephone numbers.
The Federal Communications Commission allowed telecommunication companies to compete with the incumbent local exchange carriers for services, usually by forcing the existing sole service provider to lease infrastructure to other local providers. Because of the original design of the numbering plan and the telephone switching network that assumed only a single provider, number allocations had to be made in 10,000-number blocks even when much fewer numbers were required for each new vendor. Due to the proliferation of service providers in some numbering plan areas, many area codes fell into jeopardy, facing exhaustion of numbering resources. The number blocks of failed service providers often remained unused, as no regulatory mechanism existed to reclaim and reassign these numbers.
Area codes are added by two principal methods, number plan area splits and overlay plans. Splits were implemented by dividing an area into two or more regions, one of which retained the existing area code and the other areas receiving a new code. In an overlay, multiple codes are assigned to the same geographical area, obviating the need for renumbering of existing services. Subtle variations of these techniques have been used as well, such as dedicated overlays, in which the new code is reserved for a particular type of service, such as cellular phones and pagers, and concentrated overlays, in which a part of the area retained a single code while the rest of the region received an overlay code. The only service-specific overlay in the NANP was area code 917 (New York City) when it was first installed; such service-specific area code assignments were later prohibited by the Federal Communications Commission.
Most area codes of the form N10, originally reserved for AT&T's Teletypewriter eXchange (TWX) service, were transferred to Western Union in 1969 and were freed up for other use in 1981 after conversion to Telex II service was complete. The last of these, 610, was assigned to Canada, but reassigned in 1992. These new area codes, as well as a few other codes used for routing calls to Mexico, were used for telephone area code splits in the late 1980s and early 1990s, as all other area codes under the original plan had been consumed.
After the remaining valid area codes were used up by expansion, in 1995 the rapid increase in the need for more area codes forced the NANPA to allow the digits 2 through 8 to be used as a middle digit in new area code assignments, with 9 being reserved as a last resort for potential future expansion. At the same time, local exchanges were allowed to use 1 or 0 as a middle digit. The first area codes without a 1 or 0 as the middle digit were area code 334 in Alabama and area code 360 in Washington, which both began service on January 15, 1995. This was quickly followed by area code 520 serving Arizona on March 19, 1995.
Codes ending in double digits are reserved as easily recognizable codes (ERCs), to be used for special purposes such as toll-free numbers, personal 500 numbers, Canadian non-geographic area code 600, carrier-specific 700 numbers, and high-toll 900 numbers, rather than for geographic areas. Nevada was denied 777 ("lucky 7s", a reference to the state's legalized gambling) for this reason; it received 775 instead when most of Nevada split from 702, which continues to serve the Las Vegas metropolitan area.
By 1995, many cities in the United States and Canada had more than one area code, either through splitting the city into different areas (splits) or having more than one code for the same area (overlays). For example, in Manhattan, subscribers' numbers had the NPA code 212, but two additional codes--first 917 (which initially was exclusively for cellular phones and pagers until that idea was struck down in a Federal court), then 646--were also introduced. This means that the area code must be dialed, even for local calls. In other areas, 10-digit or 11-digit dialing is now required for all local calls. The transition to 10-digit dialing typically starts with a permissive dialing phase in which 10-digit dialing is optional. During this period, the transition is heavily publicized. After a period of several months, the mandatory dialing phase is introduced, in which 7-digit dialing no longer works. Atlanta was the first U.S. city to require mandatory 10-digit dialing throughout the metropolitan area, roughly coinciding with the 1996 Summer Olympics held there. Atlanta was used as the test case not only because of its size, but also because it enjoyed the world's largest fiber optic network at the time (five times that of New York then), and it was home to BellSouth (now part of AT&T), then the Southeastern Regional Bell Operating Company.
Depending on the techniques used for area code expansion, the effect on telephone users varies. In areas in which overlays were used, this generally avoids the need for converting telephone numbers, so existing directories, business records, letterheads, business cards, advertising, and "speed-dialing" settings can retain the same phone numbers, while the overlay is used for new number allocations. The primary impact on telephone users is the necessity of remembering and dialing 10- or 11-digit numbers when only 7-digit dialing was previously permissible.
Splitting instead of overlaying generally avoids the requirement for mandatory area-code dialing, but at the expense of having to convert a region to the new code. In addition to the requirements of updating records and directories to accommodate the new numbers, for efficient conversion this requires a period of "permissive dialing" in which the new and old codes are both allowed to work. Also, many splittings involved significant technical issues, especially when the area splittings occurred over boundaries other than phone network divisions.
In 1998 area code 612, which had covered the Minneapolis - Saint Paul Twin Cities, was split to create area code 651 for St. Paul and the eastern metropolitan area. The Minnesota Public Utilities Commission mandated that the new boundary exactly follow municipal boundaries, which were distinctly different from telephone exchange boundaries, and that all subscribers keep their 7-digit numbers. These two goals were directly at odds with the reason for the split, namely to provide additional phone numbers. More than 40 exchanges had territory that straddled the new boundary. As a result, prefixes were duplicated in both area codes, which counteracted much of the benefit of the splitting, with only 200 of 700 prefixes in area 612 moving entirely to area 651. In less than two years area code 612 again exhausted its supply of phone numbers, and required a three-way split in 2000, creating the new area codes 763 and 952. Again, the split followed political boundaries rather than rate center boundaries, resulting in additional split prefixes; a few numbers moved from 612 to 651 and then to 763 in less than two years.
Recognizing that the proliferation of area codes was largely due to the telecom regulation act and the assignment of numbers in blocks of 10,000, the FCC instructed NANPA, by then administered by Neustar, to alleviate the numbering shortage. As a result, number pooling was piloted in 2001 as a system for allocating local numbers to carriers in blocks of 1,000 rather than 10,000. Because of the then design of the switched telephone network, this was a considerable technical obstacle. Number pooling was implemented with another technical obstacle, local number portability.
The program has been implemented in much of the United States by state regulators. A limited number of cities have also implemented rate center consolidation; fewer rate centers resulted in more efficient use of numbers, as carriers would reserve blocks of 1,000 or 10,000 numbers in each of multiple rate centers in the same area even if they had relatively few clients in the area. (A rate center is a geographical area used by a Local Exchange Carrier (LEC) to determine the boundaries for local calling, billing and assigning phone numbers. Typically a call within a rate center is local, while a call from one rate center to another is a long-distance call.) Together with aggressive reclamation of unused number blocks from telecom providers, number pooling has reduced the need for additional area codes, so that many previously designated area splits and overlays have been postponed indefinitely.
There is no number pooling in Canada. Number allocation remains highly inefficient as even the tiniest village is a rate center and every CLEC is assigned blocks of ten thousand numbers in every place it offers new local service. As a result, dialing seven digits even in remote locations like James Bay is more likely to produce an intercept message ("dial the area code") than an actual voice connection.
Before 1995, all NANP countries and territories outside the contiguous United States, Alaska, Hawaii and Canada shared the area code 809. This included Puerto Rico and the U.S. Virgin Islands. Each has since been assigned one or more distinct numbering plan areas; area code 809 now exclusively serves the Dominican Republic (along with area codes 829 and 849). The United States Pacific territories of the Northern Mariana Islands and Guam joined the NANP in 1997, and American Samoa became an NANP member in October 2004. The Dutch possession of Sint Maarten was originally scheduled to join the NANP on May 31, 2010, but the changeover was postponed to September 30, 2011.
|Bermuda||before 1995: served by area code 809||1995: assigned its own area code 441|
|Puerto Rico||before 1996: served by area code 809||1996: assigned its own area code 787
2001: overlaid with area code 939
|U.S. Virgin Islands||before 1997: served by area code 809||1997: assigned its own area code 340|
|Northern Marianas||before 1997: reached via IDDD using Country Code 670||1997: converted to NANPA area code 670|
|Guam||before 1997: reached via IDDD using Country Code 671||1997: converted to NANPA area code 671|
|American Samoa||before Oct. 1, 2004: reached via IDDD using Country Code 684||1997: converted to NANPA area code 684|
|Sint Maarten||before Sept. 30, 2011: reached via IDDD using Country Code 599||1997: converted to NANPA area code 721|
The NANP exhaust analysis estimates that the existing numbering system is sufficient until 2044, based on the assumption that a maximum of 667 NPAs continue to be available and that on the average 4400 central office codes are needed per year.
Various plans are discussed for expanding the numbering plan to provide relief after exhaustion. One plan being considered adds the digit 1 or 0 either to the beginning or to the end of the area code or the beginning of the seven-digit subscriber number. This would require 11-digit dialing even for local calls between any two NANP numbers. In another proposal, existing codes would be changed to x9xx, e.g. San Francisco's 415 would become 4915. Other proposals include reallocating blocks of numbers assigned to smaller long distance carriers or unused reserved services.
Of all states or territories, the U.S. state of California has the largest number of area codes assigned, followed by Texas, Florida and New York, while most countries of the Caribbean only use one. Many Caribbean codes were assigned based on alphabetic abbreviations of the territory name, as indicated in the third column of the following table (Letter code). This follows the traditional letter assignments on telephone dials. In a number of Pacific islands, the NANPA area code is the same as the International Dialing Code, which was then discontinued.
|Country/Territory||Area codes||Letter code|
|American Samoa||684||(same as IDC)|
|Antigua and Barbuda||268||ANT|
|British Virgin Islands||284||BVI|
|Canada||204, 226, ... 905|
|Dominican Republic||809, 829, 849|
|Guam||671||(same as IDC)|
|Northern Mariana Islands||670||(same as IDC)|
|Puerto Rico||787, 939||PUR (787)|
|Saint Kitts and Nevis||869|
|Saint Vincent and the Grenadines||784||SVG|
|Trinidad and Tobago||868||TNT|
|Turks and Caicos Islands||649|
|United States||201, 202, ... 989|
|United States Virgin Islands||340|
The structure of the North American Numbering Plan permits implementation of local dial plans in each plan area, depending on requirements. When multiple NPA codes serve an area in an overlay arrangement, ten-digit (10D) dialing is required. Seven-digit (7D) dialing may be permissible in areas with single area codes. Depending on the requirement of toll alerting, it may be necessary to prefix a telephone number with 1. The NANPA publishes dial plan information for individual area codes.
The standard dial plans in most cases are as follows:
|Local within area code||Local outside area code||Toll within area code||Toll outside area code|
|Single code area, with toll alerting||7D||7D or 10D||1+10D||1+10D|
|Single code area, without toll alerting||7D||1+10D||7D or 1+10D||1+10D|
|Overlaid area, with toll alerting||10D||10D||1+10D||1+10D|
|Overlaid area, without toll alerting||10D or 1+10D||1+10D||10D or 1+10D||1+10D|
The number of digits dialed is unrelated to being a local call or a toll call when there is no toll alerting. Allowing 7D local dial across an area code boundary, which is uncommon today, requires central office code protection, locally if using toll alerting, across the entire area code otherwise, to avoid assignment of the same seven-digit number on both sides.
Most areas permit local calls as 1+10D except for Texas, Georgia, and some jurisdictions in Canada which require that landline callers know which numbers are local and which are toll, dialing 10D for local calls and 1+10D for all toll calls.
In almost all cases, domestic operator-assisted calls are dialed 0+10D.
Some common special numbers in the North American system:
There are also special codes, such as:
Note: The four-digit numbers are not implemented in some areas. The codes prefixed with an asterisk (*) symbol are intended for use on Touch-Tone telephones, whereas the four-digit numbers prefixed 11xx are intended for use on rotary dial telephones, where the Touch-Tone * symbol is not available. Not all NANP countries use the same codes. For example, the emergency telephone number is not always 911: Trinidad and Tobago and Dominica use 999, as in the United Kingdom. The country of Barbados uses 211 for police force, 311 for fire, and 511 for ambulance, while Jamaica uses 114 for directory assistance, 119 for police force, and 110 for fire and ambulance services.
Despite its early importance as a share of the worldwide telephone system, few of the NANP's codes, such as 911, have been adopted outside the system. Determining that 911 requires unnecessary rotation time on rotary dial telephones, the European Union has adopted its own standardized number of 112, while countries in Asia and the rest of the world use a variety of other two- or three-digit emergency telephone number combinations. The 112 code is gaining prevalence because of its preprogrammed presence in mobile telephones that conform to the European GSM standard. The European Union and many other countries have chosen the International Telecommunication Union's 00 as their international access number instead of 011. The toll-free prefix 800 has been widely adopted elsewhere, including as the international toll-free country code. It is often preceded by a 0 rather than a 1 in many countries where 0 is the trunk prefix.
Many dials on modern telephones in use in the NANP service areas maintain the tradition of alphabetic dialing. Usually each pushbutton from digit 2 to 9 also displays three letters, which is standardized in ISO 9995-8 and, in Europe, E.161. Historically, the letters Q and Z were omitted, although some modern telephones contain them. SMS-capable devices have all 26 letters. The alphabet is apportioned to the buttons as follows:
No letters are typically mapped to keys 1 and 0, although some corporate voicemail systems use 1 for Q and Z, and some old telephones assigned the Z to the digit 0.
Originally, this scheme was used as a mnemonic device for telephone number prefixes. When telephone numbers in the United States were standardized in the mid-20th century to seven digits, the first two digits of the exchange prefix were expressed as letters rather than numbers, using the telephone exchange name. Before World War II, the largest cities used three letters and four or five numbers, while in most cities with customer dialing, phone numbers had only six digits (2L-4N). The prefix was a name, and the first two or three letters, usually shown in capital letters, were dialed. Later, the third letter, where implemented, was replaced by a digit, or an extra digit was added. This generally happened after World War II, although New York City converted in 1930. The adoption of seven-digit local numbers (2L-5N) was chosen as the requirement for direct distance dialing and progressively deployed starting the late 1940s.
The famous Glenn Miller tune PEnnsylvania 6-5000 refers to telephone number PE6-5000, a number still in service at the Hotel Pennsylvania (212 736-5000) in New York. Similarly, the classic film BUtterfield 8 is set in the East Side of Manhattan between roughly 64th and 86th Streets, where the telephone prefixes include 288. In some works of fiction, phone numbers will begin with "KLondike 5" or KLamath 5, which translates to 555, an exchange that is reserved for information numbers in North America.
The letter system was phased out, beginning before 1965, although it persisted ten years later in some places. It was included in Bell of Pennsylvania directories until 1983. Even today, some businesses still display a 2L-5N number in advertisements, e.g., the Belvedere Construction Company in Detroit, Michigan not only still uses the 2L-5N format for its number (TYler 8-7100), it uses the format for the toll-free number (1-800-TY8-7100).
Despite the phasing out of the letter system otherwise, alphabetic phonewords remain as a commercial mnemonic gimmick, particularly for toll-free numbers. For example, one can dial 1-800-FLOWERS to order flowers, or 1-800-DENTIST to find a local dentist.
In addition to commercial uses, alphabetic dialing has occasionally influenced the choice of regional area codes in the United States. For example, when area 423 (East Tennessee) was split in 1999, the region surrounding Knoxville was assigned area code 865, chosen to represent the word VOL (Volunteers), the nickname of Tennessee (The Volunteer State), as well as athletic teams at the University of Tennessee. Another example of this is area code 859 in Kentucky, which was chosen to represent 'UKY' as a nod to the University of Kentucky in Lexington, KY, the code's principal city.
Several Caribbean area codes were chosen as an alphabetic abbreviation of the country name, which are indicated in the table of NANP regions.
The North American Numbering Plan does not set aside special non-geographic area codes exclusively for cellular phones. Only one regional exception exists in area code 600 in Canada. In many other national numbering plans outside the NANP, mobile services are assigned separate prefixes. Cell phone numbers in the NANP are allocated within each area code from special central office prefixes and calls to them are billed at the same rate as any other call. Consequently, the caller pays pricing model adopted in other countries, in which calls to cellular phones are charged at a higher nationwide rate, but incoming mobile calls are not charged to the mobile user, could not be implemented. Instead, North American cellular telephone subscribers are also generally charged for receiving calls (subscriber pays). This has discouraged mobile users from publishing their telephone number. However, price competition among carriers has reduced the average price per call minute for contract customers for both inbound and outbound calls, which compare favorably to those in caller-pays countries. Most users select bundle pricing plans that include an allotment of minutes expected to be used in the billing period. Of the four major national carriers in the United States, all four (AT&T, Sprint, T-Mobile, Verizon) offer free calling between mobile phones on the carrier's network, and Sprint also offers its customers free calling to mobile phones on other networks.
Industry observers have attributed the relatively low mobile phone penetration rate in the United States, compared to that of Europe, to the subscriber-pays model. In this model the convenience of the mobility is charged to the subscriber. Callers from outside the local-calling region of the assigned number, however, pay for a long-distance call, although domestic long-distance rates are generally lower than the rates in caller-pays systems. Conversely, an advantage of caller-pays is the relative absence of telemarketing and nuisance calls to mobile numbers. The integrated numbering plan also enables local number portability between fixed and wireless services within a region, allowing users to switch to mobile service while keeping their telephone number.
The initial plan for area code overlays did allow for providing separate area codes for use by mobile devices, although these were still assigned to a specific geographical area, and were charged at the same rate as other area codes. Initially, the area code 917 for New York City was specifically assigned for this purpose within the boroughs; however, a Federal court overturned the practice and the use of an area code for a specific telephony purpose. Since mobile telephony has been expanding faster than landline use, new area codes typically have a disproportionately large fraction of mobile and nomadic numbers, although landline and other services rapidly follow and local network portability can blur these distinctions.
The experience of Hurricane Katrina and similar events revealed a possible disadvantage of the methods employed in the geographic assignment of cellular numbers. Many mobile phone users could not be reached, even when they were far from the stricken areas, because the routing of calls to their phones depended on equipment in the affected area. They could make calls but not receive them.
The use of geographic numbers may also lead to tromboning; one can take a handset with a Vancouver number into St. John's and outbound calls to St. John's numbers while in that city will be local, but incoming calls must make the cross-country trip to Vancouver and back. This adds costs for subscribers, as an 8,000 km cross-country call (as a worst case) incurs long-distance tolls in both directions. AMPS subscribers used to be provided with a local number (such as 1-NPA-NXX-ROAM) in each city, allowing them to be reached by dialing that number plus the ten-digit mobile telephone number; this is no longer supported.
Calls between different countries and territories of the NANP are not typically charged at domestic rates. For example, most long-distance plans may charge a California subscriber a higher rate for a call to British Columbia than for a call to New York, even though both destinations are within the NANP. Similarly, calls from Bermuda to U.S. numbers (including 1-800 numbers, which are normally thought of as toll-free) incur international rates. This is because many of the island nations implemented a plan of subsidizing the cost of local phone services by directly charging higher pricing levies on international long-distance services.
Because of these higher fees, scams had taken advantage of customers' unfamiliarity with pricing structure to call the legacy regional area code 809. Some scams lured customers from the United States and Canada into placing expensive calls to the Caribbean, by representing area code 809 as a regular domestic, low-cost, or toll-free call. The split of 809 (which formerly covered all of the Caribbean NANP points) into multiple new area codes created many new, unfamiliar prefixes which could be mistaken for U.S. or Canada domestic area codes but carried high tariffs. In various island nations, premium exchanges such as +1-876-HOT-, +1-876-WET- or +1-876-SEX- (where 876 is Jamaica) became a means to circumvent consumer-protection laws governing area code 900 or similar U.S.-domestic premium numbers.
These scams are on the decline, with many of the Cable and Wireless service monopolies being opened up to competition, hence bringing rates down. Additionally, many Caribbean territories have implemented local government agencies to regulate telecommunications rates of providers.
The Telecommunications Act of 1996 (47 U.S.C. § 251 (b)(2)) authorizes the Federal Communications Commission (FCC) to require all local exchange carriers (LECs) to offer local number portability. The FCC regulations were enacted on June 27, 1996, and changes were to take effect in the 100 largest Metropolitan Statistical Areas by October 1, 1997 and elsewhere by December 31, 1998.
The North American Numbering Council (NANC) was directed to select the Local Number Portability Administrators (LNPAs), also referred to as the Number Portability Administrator, akin to the North American Numbering Plan Administrator (NANPA).
American television programs and films often use the central office code 555, or KLamath 5 and KLondike 5 in older movies and shows, for fictitious telephone numbers, to prevent disturbing actual telephone subscribers if anyone is tempted to dial a telephone number seen on screen.
Occasionally, valid telephone numbers are used in contexts such as songs with varying intents and consequences. An example is the 1981 song "867-5309/Jenny" by Tommy Tutone, which is the cause of a large number of calls.
Not all numbers beginning with 555 are fictional. For example, 555-1212 is the standard number for directory assistance. Only 555-0100 through 555-0199 are reserved for fictional use. Where used, these are often routed to information services; Canadian telephone companies briefly promoted 555-1313 as a pay-per-use "name that number" reverse lookup in the mid-1990s.