List of North American Numbering Plan area codes  The North American Numbering Plan (NANP) divides the regions of its member nations into geographic numbering plan areas (NPAs). Each area is assigned a three-digit area code which is prefixed to each telephone number issued in its service area. Some areas are assigned multiple codes in an overlay plan. The rules for numbering NPAs do not permit the digits "0" and "1" as the leading digit, which also applies for central office codes. "0" has been used traditionally for operator-assisted telephone calls, and the digit "1" was traditionally ignored in switching equipment as a leading digit, because it could not be distinguished reliably from intermittent loop disconnections. NPAs with "9" as the second digit are reserved for future format expansion. Area codes with identical two trailing digits, such as "322" and "800", are considered easily recognizable codes (ERC), of which the series "8xx" is used for toll-free services. Future area codes are written in "italics".