Area codes 301, 240, and 227

By the end of the 1980s, the Washington metropolitan area was running out of assignable prefixes for new central offices. The only unassigned prefixes were unavailable due to the central office code protection that maintained seven-digit dialing in the metro-area. Assignment of these prefixes would require dialing an area code. The three local operating companies of the Chesapeake and Potomac Telephone Company (C&P Telephone, later part of Bell Atlantic and now Verizon) cooperated in ended code protection on October 1, 1990, for entire area and required ten-digit dialing for all local calls between Maryland, the District of Columbia, and Virginia.[2] Area code 202 was no longer usable for suburban points. Local calls within Maryland did not require the area code.[3] Permissive dialing using the old dialing procedures continued from April 1 through October 1, 1990.[4]

Despite the overall growth of the Baltimore–Washington metropolitan area, 301 remained the exclusive area code for Maryland for 44 years, making Maryland one of the largest states with a single area code. By the end of the 1980s, it became apparent that terminating central office code protection in the Washington area could not free up sufficient resources north of the Potomac River to stave off the immediate need for a new area code.

In mitigation action, Baltimore and the Eastern Shore were reassigned from area code 301 to area code 410 on November 1, 1991, to form a new numbering plan area.[5][6] The area code split largely followed metropolitan area lines. There were a few exceptions, such as in Howard County, which is often recognized as part of the Baltimore metropolitan area, a small portion of the county remained in 301, while the rest of the county was reassigned to 410.[7]

When an area code is split, the largest city, in this case Baltimore, in the old numbering plan area typically retains the existing area code to minimize expense for changing telephone numbers. In this case, Bell Atlantic spared the large number of federal agencies on the Maryland side of metropolitan Washington from the expense and disruption.

Although the area code split was intended as a long-term solution, within four years 301 was close to exhaustion due to the proliferation of cell phones and pagers, especially in the Washington suburbs. In relief action, area code 240 was introduced on June 1, 1997, to form the state's first overlay.[8] Overlays were a new concept at the time, and met with some resistance due to the requirement for ten-digit dialing.[9] It was decided to create an overlay rather than split the region into two numbering plan areas to avoid assigning a new area code to 1.2 million existing telephone numbers.[10] At the time, it was predicted that it would take until 2008 to exhaust area code 240.[11]

A September 2022 study projected that the 301/240 numbering plan area (NPA) would suffer central office code exhausted between April and June 2023.[12] Area code 227 was already approved as an all-services distributed overlay since January 3, 2001 for the 301/240 NPA.[13][14] The new area code went into service on June 14, 2023, at which time activation of new central office codes could begin, once all assignable central office codes in 240/301 had been exhausted.