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An increasing demand for telephone numbers has existed since the development of automatic telephony in the early 20th century, but was spurred especially since the 1990s, with the proliferation of fax machines, pager systems, mobile telephones, computer modems and, eventually, [[smart phone]]s.
The implementation of an area code split typically involves the establishment of a [[Class-4 telephone switch|Class-4 toll switching center]] for each division of the existing numbering plan area that receive a new area code. The local seven-digit telephone numbers in any of the areas are typically not changed. The existing central office prefixes are maintained and only the central offices of the new divisions are reassigned to a new area code. The impact of a split on the general public involves
Since area code splits have substantial impact in the involved communities, and involve substantial cost in telephone plant and exchange equipment, they are planned carefully well ahead of implementation with the intent that an area is not again affected by a subsequent realignment for at least a decade.<ref name="notes1975">AT&T, ''Notes on the Network'' (1975), Section 2</ref>
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The new boundaries of the numbering plan areas are drawn in a manner that minimizes splitting communities and should coincide with political subdivision where practical.<ref name="notes1975" /> Other geographic features, such as rivers and bodies of water, mountain ranges, or highways may serve as guides for boundary placements. Tributary toll telephone routes should not be unduly cut, so prevent rerouting to new toll center switching systems.
The area that retains the existing area code is typically the largest, or historically more established or developed place, but the locations of large government bodies or other criteria may
==Area code overlays==
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Not withstanding the desire for long-term stability of the local numbering plan and customer understanding, rapid growth in some areas has resulted in many splits within just a few years.
As a result, in the early 1990s, the [[North American Numbering Plan Administrator]] (NANPA) introduced another method for exhaustion relief: the area code overlay. This method assigns multiple area codes to the same numbering plan area, so that existing subscribers can keep established telephone numbers. Only new accounts and extra lines receive telephone numbers with the new area code. This method requires [[ten-digit dialing]] of local calls for customers of both area codes
==See also==
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