Carrier identification codes are four-digit codes in the format XXXX, where X is any digit from 0 through 9. These codes are assigned by the North American Numbering Plan to local exchange carriers that provide long distance call services.
Carrier identification codes mainly help in routing and billing of long distance calls within the NANP telecommunication network. When user dials a long distance call he can choose a specific carrier by prefixing the number dialed by 101XXXX, where XXXX is the carrier identification code. For example if the carrier identification code is 9872, then the user should prefix his number with 1019872.