[2] The collection of permissible digit patterns, so called digit-maps, for a private telephone system or for customer premise equipment, such as an analog telephone adapter (ATA) or an IP phone, is sometimes also called dial plan.
PBX equipment, carrier switching systems, and end-user telephones may use variable-length or fixed-length dial plans.
In private branch exchanges in the U.S. a dialing plan may specify the addresses of internal extensions, typically as numbers of two, three, or four digits.
Analog telephone adapters, IP phones, and many other VoIP media gateways have configuration options that establish the digit sequences that can be dialed with the equipment.
The following syntax may be used for such dial plan, as adapted from RFC 2705,[3] the specification for the Media Gateway Control Protocol.