A recent innovation in the USA has been that of the area code overlay scheme.  Whereby city/area codes were traditionally assigned in a linear geographic fashion, the overlay method implemented in the late 1990s has made it possible for numerous area codes to co-exist in the same region.  This system has been employed in several metropolitan areas in the US and is also used in the Toronto area of Eastern Canada.  Unfortunately, the use of overlays can make the task of discerning a particular code used in a certain city or area more difficult.

The Area Codes in use are too numerous to list here, and codes are changed or added regularly. Areas scheduled for or undergoing changes can be found at http://areacode-info.com/.  The links below can help you find an Area Code or phone listing for a particular city or state.