Area code 301 was one of the original area codes established in 1947 when the NANP was created. Its original numbering plan area comprised the entire state of Maryland. From 1947 to 1990, the telephone numbering plan of the entire Washington metropolitan area, including Washington, D.C., and the large suburban areas in Maryland and Virginia, operated in a system of central office code protection, meaning that no central office code in Washington D.C., and in the Maryland and Virginia suburbs could exist in more than a single central office. As a result, it was possible for telephone users in the metropolitan area to dial any other telephone number in the area as a local call with only seven digits, without using an area code. The entire metro area was reachable via long-distance services by dialing area code 202, for which purpose AT&T had established cross-referenced operator routing codes for all affected central offices. For example, if 202-574 numbers were in use in the district or 703-574 numbers were in use in Northern Virginia, the corresponding 301-574 numbering block could only be assigned in areas considered a safe distance away from the capital, such as the Eastern Shore of Maryland. By the end of the 1980s, the Washington metropolitan area was running out of prefixes. This code exhaustion was mitigated on October 1, 1990, by removing code protection in 202. As of that date, all local metro area calls between Maryland, the District of Columbia, and Virginia required dialing the area code for calls to another NPA. Area code 202 was no longer useable for suburban points. Local calls within Maryland did not require the area code. 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, however, it became apparent that breaking seven-digit dialing in the Washington area would not free up enough numbers to stave off the need for a new area code. Baltimore and the Eastern Shore were split off as a new numbering plan area with area code 410 on October 6, 1991. The split largely followed metro area lines. However, part of Howard County, which is recognized as part of the Baltimore area, stayed in 301, while the rest shifted to 410. Normally, when an area code is split, the largest city in the old numbering plan area retains the existing area code—in this case, Baltimore. However, it was decided to let the Washington suburbs keep 301. Not only do the Washington suburbs have the bulk of the state's population, but Bell Atlantic wanted to spare the large number of federal agencies on the Maryland side of the metro the expense and burden of having to change their numbers. This was intended as a long-term solution, but within four years 301 was close to exhaustion due to the proliferation of cell phones and pagers, especially in the Washington suburbs. To solve this problem, area code 240 was introduced on June 1, 1997, as the state's first overlay area code. Overlays were a new concept at the time, and met with some resistance due to the requirement for ten-digit dialing. For this reason, conventional wisdom would have suggested a split in which the Washington suburbs would have kept 301 while Frederick and points west would have shifted to 240. However, Bell Atlantic wanted to spare residents the burden of having to change their numbers. Number exhaustion of the overlay complex 301/240 is not expected before 2025. However, the North American Numbering Plan Administrator has assigned area code 227 for further overlay relief action when needed.
