Overlay plan
METHOD OF AREA CODE RELIEF FOR AN EXHAUSTED CODE
Area code overlay; Overlay area code; Overlay complex
In telecommunications, an overlay plan is a telephone numbering plan that assigns multiple area codes to a geographic numbering plan area (NPA). Overlaying numbering plan area codes is practiced in territories of the North American Numbering Plan (NANP) to prevent exhaustion of telephone numbers and central office codes in growth areas.