Overlay complex

In telecommunications, an area code overlay complex is a telephone numbering plan that assigns multiple area codes to a geographic numbering plan area (NPA). Area code overlays are implemented in territories of the North American Numbering Plan (NANP) to mitigate exhaustion of central office codes in growth areas. The method has been in use since 1992, and has been the preferred and exclusive method of relief since 2007. From the inception of the North American Numbering Plan in 1947 until 1992, the only method of introducing new area codes was the area code split. It divided an existing numbering plan area (NPA) into multiple regions, one of which retained the existing area code. The other parts were assigned a new NPA code each. The retaining area was usually the historically more established or developed place, and required no numbering changes. It gained numbering capacity by acquiring the central office codes of the other areas, because those were delegated to a new area code in the process. The existing central office codes in the separated areas are maintained in the new numbering plan area, so that the only change for existing subscribers is the change of area code. Thus, only area code references, where present for long-distance calling, required updating in directories, letterheads and business cards. With the proliferation of electronic central office switching starting in the 1980s, it became possible to implement another method for expanding numbering plan resources, the area code overlay. In an overlay numbering plan, also called overlay complex, an additional area code is assigned to an existing numbering plan area, thereby avoiding the need for existing customers in the reassigned regions to change telephone numbers. The first use of that solution was in New York City in 1992, when area code 917 was overlaid to two numbering plan areas, Manhattan's area code 212, and area code 718 in the outer boroughs. Most implementations of the overlay method have required ten-digit dialing, by including the area code, for all subscribers for all calling destinations, local or long-distance.