North American Numbering Plan expansion

The expansion of the North American Numbering Plan (NANP) is an anticipated requirement to provide more telephone numbers to accommodate future needs beyond the pool of ten-digit telephone numbers in use since the inception of the NANP in 1947.

The North American Numbering Plan was devised in 1947, and has provided customer direct-dialed long-distance telephone service since 1951. Occasionally, the growing demand for telephone numbers has mandated certain changes in the format of telephone numbers and method by which they are allocated to telephone companies. Instead of expanding the size of telephone numbers, as implemented in many countries outside North America, the NANP was modified to allow continued use of seven-digit telephone numbers and a three-digit area code. The first modifications removed restrictions placed on the central office prefixes that existed due to the incorporation of telephone exchange names into the numbering plan. Later modifications relaxed the restrictions for the digits permitted in the area code format.

Every place in the countries and territories of the NANP already belongs to a numbering plan area (NPA). Assigning a new area code requires either geographically splitting an existing numbering plan area (split plan) and reassigning the subscribers in the new, smaller area to a new area code, or assigning an additional area code to an existing numbering plan area, which is called an overlay plan. The first new area codes were geographic splits, until the geographical areas were at risk of becoming unreasonably small. Since then, most new area codes are overlaid. When a numbering plan area has more than one area code, telephone companies usually can no longer support dialing seven-digit local numbers, and require ten-digit dialing for all calls within the geographical area. Imposing ten-digit dialing for local calls effectively converts every local number in these geographical areas into a ten-digit local number. Universal ten-digit local numbering is arguably inevitable, as more geographical areas get overlaid.

Demand for telephone numbers continues to grow. It will soon be necessary to reduce (or discard) the geographic association of "area codes" and begin assigning new users to arbitrary ten-digit phone numbers, or else finally change from the strict 10-digit format to something else (10-or-11 digits, 11 digits, or 12 digits).

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