All phone numbers in the North American Numbering Plan (NANP) are allocated first as area codes by region and then by prefix for smaller locations. Some states

All phone numbers in the North American Numbering Plan (NANP) are allocated first as area codes by region and then by prefix for smaller locations. Some states, like Wyoming, have a single area code (307) while metro areas, like Houston, can have multiple area codes (713, 281, 832, 346) all covering the same physical place. This is a holdover of calling party pays so that a caller will know where a given phone number "lives." This distinction has become less visible with the advent of local number portability and nationwide mobile roaming. (The NANP covers Canada, the Northern Mariana Islands, the United States, and a large list of island nations in the North American area.)