NANP

The North American Numbering Plan (NANP) is an integrated telephone numbering plan of 24 countries and territories: the United States and its territories, Canada, Bermuda, and 16 Caribbean nations. It is a system of three-digit area codes and seven-digit telephone numbers that directs telephone calls to particular regions on a public switched telephone network (PSTN), where they are further routed by the local network.
See more at Wikipedia.org...