Overview#Telephone Numbering Plan is a type of numbering scheme used in telecommunication to assign Phone Numbers to subscriber telephones or other telephony endpoints.
Telephone Numbering Plans are defined in each of administrative regions of the Public Switched Telephone Network (PSTN) and they are also present in private telephone networks. For public number systems, geographic location plays a role in the sequence of numbers assigned to each telephone subscriber.
Telephone Numbering Plans may follow a variety of design strategies which have often arisen from the historical evolution of individual telephone networks and local requirements. A broad division is commonly recognized, distinguishing open numbering plans and closed numbering plans. Many numbering plans subdivide their territory of service into geographic regions designated by a prefix, often called area code, which is a set of digits forming the most-significant part of the dialing sequence to reach a telephone subscriber.
The International Telecommunication Union (ITU) has established a comprehensive numbering plan, designated E.164, for uniform interoperability of the networks of its member state or regional administrations. It is an open numbering plan, however, imposing a maximum length of 15 digits to Phone Numbers. The E.164 standard defines a Country Calling Code for each state or region which is prefixed to each national numbering plan telephone number for international destination routing.
Private numbering plans exist in telephone networks that are privately operated in an enterprise or organizational campus. Such systems may be supported by a Private Branch Exchange (PBX) which controls internal communications between telephone extensions.
Telephone Numbering Plans also decide on the routing of Signalling System No. 7 (SS7) signaling messages as part of the Global Title. In Public Land Mobile Networks, the E.212 numbering plan is used for Subscriber identities, e.g., stored in the GSM SIM, while E.214 is used for routing database queries across PSTN networks. In general, the structure of telephone numbers issued within a national Telephone Numbering Plan follows both the international formats and the national standards. Within the international system administered by the ITU in E.164, each national plan has a unique Number Plan Area.