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An area code overlay is a numbering plan area (NPA) in the North American Numbering Plan (NANP) that has multiple area codes assigned. Overlay plans are used to increase the number of available telephone numbers in a province, state, or region. The North American Numbering Plan Administrator (NANPA) maintains the definitive list of overlays in the United States.The majority of overlays have occurred since 1995. The process has been driven by fragmentation of numbering resources by a growing number of competitive local exchange carriers.