 The North American Numbering Plan was devised in the 1940s, was implemented 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 the 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 designed as a closed numbering plan, having a fixed number of digits, three digits for the numbering plan area (NPA) code and seven digits for telephone numbers that are unique in each NPA.