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The combination of an area code and the telephone number serves as a destination routing address in the public switched telephone network (PSTN). The telephone number consists of a three-digit central office code and a four-digit station number. Each telephone is assigned a seven-digit telephone number unique only within its respective numbering plan area. The NANP divides the territories of its members into numbering plan areas ( NPAs) which are encoded numerically with a three-digit telephone number prefix, commonly called the area code. Canadian numbering decisions are made by the Canadian Numbering Administration Consortium. Each participating country forms a regulatory authority that has plenary control over local numbering resources. AT&T continued to administer the numbering plan until the breakup of the Bell System, when administration was delegated to the North American Numbering Plan Administrator (NANPA), a service that has been procured from the private sector by the Federal Communications Commission (FCC) in the United States. The NANP was originally devised in the 1940s by the American Telephone and Telegraph Company (AT&T) for the Bell System and the independent telephone operators in North America to unify the diverse local numbering plans that had been established in the preceding decades and prepare the continent for direct-dialing of calls by customers without the involvement of telephone operators. Some North American countries, most notably Mexico, do not participate in the NANP. This group is historically known as World Zone 1 and has the international calling code 1.
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The North American Numbering Plan ( NANP) is a telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. For other uses, see Nanpa (disambiguation).