In North America, phone number prefixes are assigned by the North American Numbering Plan, which is administered by the North American Numbering Plan Administration. Most of the rest of the worlds prefixes are assigned by the International Telecommunication Union.
Phone numbers in the NANP consist of 10 digits. The first three numbers consist of the Numbering Plan Area code, which is more commonly referred to simply as the area code. A seven-digit local number follows the area code. The phone number prefix consists of the first three digits of the seven-digit local number.
The NANP was developed in 1947 and implemented in 1951 by AT&T. Twenty-five countries and territories participate in the North American Numbering Plan including the United States, Canada and many countries in the Caribbean.
The NANP was created to facilitate the direct dialing of long distance calls. Prior to the development of the NANP, phone calls were routed through a public switchboard,with the phone number's prefix indicating the central office code. Due to the advancement in phone technology, however, the phone number prefix is no longer necessarily tied to a geographical location. The telephone number prefix 555 is reserved in most US telephone areas for special services or fictitious uses.