A Carrier Access Code, or CAC, is a seven-digit code that allows callers to reach a specific carrier from any telephone. These codes are sometimes used to dial around an interexchange carrier and obtain better rates. All carrier access codes start with 101 followed by the carrier identification code.
Carrier identification codes are defined by the North American Numbering Plan Administrator and are used to identify, bill and route telecommunications traffic. A list of all available carrier identification codes is available online. U.S. mobile carriers are not required to support carrier access codes, and dialing one from a cell phone may not be possible.