Dial plan
A dial plan (or dialing plan) establishes the permitted sequences of digits dialed on subscriber lines or on subscriber premises equipment, such as telephones and private branch exchange (PBX) systems. Dial plans in the public switch telephone network (PSTN) have traditionally been more commonly referred to as "dialing procedures". 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". A pattern may be as short as a single digit, e.g. for  reaching an operator, or as long as a complete international telephone number, including trunk prefixes and international prefixes. Public switched telephone network (USA). Similarly, telephony service operators may provide dialing sequences for special services, such as directory assistance and emergency services. Private telephone systems. PBX equipment, carrier switching systems, or end-user telephones may specify a variable-length dial plan or a fixed-length dial plan. In private branch exchanges in the U.S. a dialing plan may specify the addresses of internal extension, as numbers of two, three, or four digits. Digit maps. 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 dial plan of these devices is established by a digit map. The following syntax may be used for such dial plan, as adapted from RFC 2705, the specification for the Media Gateway Control Protocol. Some "dial plan" examples using the above syntax look as follows: