In telecommunications, an overlay numbering plan is the practice of introducing a new area code by assigning it to the same geographic area of an already existing area code. This results in areas within which telephone numbers exist in multiple area codes. Overlaying area codes is predominantly practiced in the territories belonging to the North American Numbering Plan (NANP).
Prior to the introduction of overlay plans, new area codes were introduced by dividing an existing numbering plan area (NPA) into multiple regions. One of these regions, usually the historically more established or developed one, retains the existing area code, requiring no numbering changes in that area, but making available in that area the central office codes of the other regions, and thus enlarging the number pool. However, all subscribers in the newly assigned area are required to update telephone number references, such as on letter heads, business cards, and in directories. For example, the original area code for the entire state of Washington was 206; today 206 applies to only the city of Seattle and the immediate vicinity. This practice became known as a split plan.
In an overlay numbering plan, the change of the area code for numbers in parts of the existing numbering plan area is avoided by assigning additional area codes to the entire region of an existing code. The first use of this solution was in the borough of Manhattan in New York City, where area code 917 was added to the original 212.
In several cases, overlay plans were implemented on a special case basis to implement specialized dialing plans. In some areas, a party in one area code could dial an office prefix which was local, but in a different area code, with only 7 digits. If they were calling a distant office prefix in the same area code, they would either have to dial 1 and the number or 1+area code+number.
This practice was implemented on a large scale in Washington, D.C. and its suburbs in Maryland and Virginia. Until 1991, the entire region was a single local calling area, and it was possible for anyone living in the metro area to dial a number in D.C. or the Maryland and Virginia suburbs with only seven digits. This setup was possible because the entire Washington metropolitan area is a single local access and transport area (LATA). Every number in Maryland's area code 301 and northern Virginia's 703 was given a "hidden" phone number consisting of the same number in the D.C.'s area code 202, essentially making area code 202 an overlay for the entire region. This meant a Virginia number, such as 703-931-xxxx, could also have been dialed as 202-931-xxxx, while a Maryland number, such as 301-585-xxxx, could also be dialed as 202-585-xxxx. However, this also implied that no central office code could be duplicated in any of the three territories, even in areas a safe distance from the metro area such as southwestern Virginia or the Eastern Shore of Maryland. By 1991 the use of 202 as a ''de facto'' overlay was discontinued due to growing demand for numbers, and callers in the area dialing an out-of-area-code number had to dial the full 10 digit number.
A similar scheme was employed in Canada's capital, Ottawa. It shares the same calling area as Hull, its twin city in Quebec, even though Ottawa was in area code 613 and Hull was in area code 819. Until 2006, it was possible to place a call between Ottawa and Hull with only seven digits. This continued even after Hull was merged into the larger city of Gatineau in 2002. As in the Washington area, this was implemented in a way that the same number could not be duplicated anywhere in eastern Ontario or western Quebec. However, Canada's inefficient number allocation system (see below) and the proliferation of cell phones brought 819 to the brink of exhaustion by the turn of the century. The only available numbers in 819 were numbers that could have theoretically been used in the former Hull, but could not be issued without breaking seven-digit dialing between Ottawa and Hull. The scheme was largely ended in 2006. The sole legacy of the old system is a "dual dialability" system for federal government numbers on both sides of the provincial border; all federal government offices on the Quebec side duplicated several exchanges worth of their counterparts on the Ontario side.
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