A telephone numbering plan is a type of numbering scheme used in a telecommunications network to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined world-wide, as well as within each of the administrative regions of the public switched telephone network (PSTN), and in private telephone networks. In public numbering systems, geographic location typically plays a role in the sequence of numbers assigned to each telephone subscriber. Many numbering plan administrators subdivide their territory of service into geographic regions designated by a prefix, often called an area code or city code, which is a set of digits forming the most-significant part of the dialing sequence to reach a telephone subscriber. Within such regions designated by area codes, locally unique telephone numbers are assigned based on locally determined principles but in agreement with the larger-network rules. 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 between closed and open numbering plans. A closed numbering plan, as found in North America, features fixed-length area codes and local numbers, while an open numbering plan has a variance in the length of the area code, local number, or both of a telephone number assigned to a subscriber line. The latter type developed predominantly in Europe. 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 but imposes a maximum length of 15 digits to telephone numbers. The standard defines a country code for each member region which is prefixed to each national 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 provides a central access point to the PSTN and also controls internal calls between telephone extensions. In contrast to numbering plans which determine telephone numbers assigned to subscriber stations, dialing plans establish the customer dialing procedures, i.e., the sequence of digits or symbols to be dialed to reach a destination. It is the manner in which the numbering plan is used. Even in closed numbering plans, it is not always necessary to dial all digits of a number. For example, an area code may be omitted when the destination is in the same area as the calling station and the destination has only one area code.
Telephone number structure National or regional telecommunication administrations that are members of the International Telecommunication Union (ITU) use national telephone numbering plans that conform to international standard E.164. E.164 specifies that a telephone number consists of a country code and a national telephone number. National telephone numbers are defined by national or regional numbering plans, such as the European Telephony Numbering Space, the North American Numbering Plan (NANP), or the UK number plan. Within a national numbering plan, a complete destination telephone number is typically composed of an area code and a subscriber telephone number. Many national numbering plans have developed from local historical requirements and progress or technological advancements, which resulted in a variety of structural characteristics of the numbers assigned to telephones. In the United States, the industry decided in 1947 to unite all local telephone networks under one common numbering plan with a fixed length of ten digits for the national telephone number of each telephone, of which the last seven digits were known as the local directory number, or subscriber number. Such a numbering plan became known as a closed numbering plan. In several European countries, a different strategy prevailed, known as the open numbering plan, which features a variance in the length of the area code, the local number, or both.
Subscriber number The subscriber number is the address assigned to a telephone line or wireless communication channel terminating at the customer equipment. The first few digits of the subscriber number may indicate smaller geographical scopes, such as towns or districts, based on municipal aspects, or an individual telephone exchange (central office code), such as a wire center. In mobile networks they may indicate the network provider. Callers in a given area do not need to include area prefixes when dialing within the same area if the area has only one area code. Devices that dial telephone numbers automatically may include the full number with area and access codes. The subscriber number is typically listed in local telephone directories, and is therefore often referred to as the directory number.
Area code
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