Area codes 410, 443, and 667 are telephone area codes serving the eastern half of the U.S. state of Maryland, including the Baltimore metropolitan area and the Eastern Shore. The 410 area code is the main area code, while the 443 and 667 codes are overlay codes. 443 and 667 were primarily used with cell phones and CLEC carriers such as Comcast or Cavalier Telephone when introduced but have since become universal in their carrier availability.
Before these area codes were created, all of Maryland had been served by 301 since the institution of area codes in 1947, even though the state is home to two very large metropolitan areas--Baltimore and the Maryland suburbs of Washington, D.C. (area code 202). This made Maryland one of the most-populous states to be served by a single area code. However, by the late 1980s, 301 was on the verge of exhaustion due to the rapid growth of fax machine usage and the Baltimore and Washington suburbs. The supply of available numbers was further limited by the fact that most of the Maryland side of the Washington area shares an LATA with northern Virginia and the District itself. For over 40 years, it was possible to make a call between portions of the Washington area with only seven digits, since every number in 301 and northern Virginia's 703 was given a "hidden" number in the District's area code 202.
Plans had already been underway to break this longstanding scheme, but it soon became apparent that this would not free up enough numbers to keep up with demand. It was now obvious that a second area code was necessary. In November 1990, a plan to add a second area code, the 410 code, to the state was announced. It was decided that the Baltimore metropolitan area and the Eastern Shore would get the new area code, while western and southern Maryland--including the Washington suburbs--would remain with the 301 area code.
Area code 410 officially entered service on October 6, 1991; it was initially implemented in permissive-dialing mode, not as a flash-cut, with the 10-digit dialing scheme coming in for local calls across the new 301/410 boundary. The split largely followed metro lines. However, part of Anne Arundel, Carroll County, Frederick County, and Howard County were split between area codes 301 and 410.[a] Effective November 1, 1991, ten-digit dialing was required when calling a different area code in Maryland.
Although the split was intended to be a long-term solution, within five years 410 was already close to exhaustion due to the area's rapid growth and the proliferation of cell phones and pagers. To solve this problem, area code 443 was overlaid onto the 410 territory on July 1, 1997. Overlays were a new concept at the time, and had met with some resistance due to the prospect of different area codes in the same area as well as the requirement for ten-digit dialing. However, Bell Atlantic (now Verizon), the dominant carrier in the region, wanted to keep residents from having to change their numbers for the second time in less than a decade.
By 2011, the 410/443 area was once again running out of numbers due to the proliferation of cell phones. To prevent residents from having to change their phone numbers to a new area code, the overlay of area code 667 was implemented on March 24, 2012.
The counties served by these area codes include:
In the Baltimore metropolitan area: