Overlay plan

In telecommunications, an overlay numbering plan is the practice of introducing a new area code by assigning it to an existing numbering plan area (NPA) that already has an area code assigned. This results in areas with seven-digit telephone numbers that exist with multiple area codes. Overlaying area codes is practiced in the territories belonging to the North American Numbering Plan (NANP).

Starting with the creation of the North American Numbering Plan in 1947, new area codes were introduced by dividing an existing numbering plan area into multiple regions. One of these regions, usually the historically more established or developed place, retains the existing area code, requiring no numbering changes in that area. This makes available in that area the central office codes of the other parts of the old numbering plan area, 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 specialised 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.

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