Overlay plan

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).

Methodology

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.