Telephone numbering plan
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, 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 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, 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 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, 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, 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
Telephone administrations that manage telecommunication infrastructure of extended size, such as a large country, often divide the territory into geographic areas. This benefits independent management by administrative or historical subdivisions, such as states and provinces, of the territory or country. Each area of subdivision is identified in the numbering plan with a routing code. This concept was first developed in the planning for a nationwide numbering plan for Operator Toll Dialing and Direct Distance Dialing in the Bell System in the United States in the 1940s, a system that resulted in the North American Numbering Plan for World Zone 1. AT&T divided the United States and Canada into numbering plan areas, and assigned to each NPA a unique three-digit prefix, the numbering plan area code, which became known in short-form as NPA code or simply area code. The area code is prefixed to each telephone number issued in its service area.Other national telecommunication authorities use various formats and dialing rules for area codes. The size of area code prefixes may either be fixed or variable. Area codes in the NANP have three digits, while two digits are used in Brazil, and one digit is used in Australia and New Zealand. Variable-length formats exist in many countries, including Argentina, Austria, Germany, Japan, Mexico, and the United Kingdom.
In addition to digit count, the format may be restricted to certain digit patterns. For example, the NANP had at times specific restrictions on the range of digits for the three positions of the area code, and required assignment to geographical areas that avoided nearby areas receiving similar area codes, to avoid confusion and misdialing.
Some countries, such as Denmark and Uruguay, have merged variable-length area codes and telephone numbers into fixed-length numbers that must always be dialed regardless of location. In such administrations, the area code is not distinguished formally in the telephone number.
In the UK, area codes were first known as subscriber trunk dialling codes. Depending on local dialing plans, they are often necessary only when dialed from outside the code area or from mobile phones. In North America, ten-digit dialing is required in areas with overlay numbering plans, in which multiple area codes are assigned to the same area.
The strict correlation of a telephone number to a geographical area has been broken by technical advances, such as local number portability in the North American Numbering Plan and voice over IP services.
When dialing a telephone number, the area code may have to be preceded by a trunk prefix or national access code for domestic calls, and for international calls by the international access code and country code.
Area codes are often quoted by including the national access code. For example, a number in London may be listed as 020 7946 0321. Users must correctly interpret 020 as the code for London. If they call from another station within London, they may merely dial 7946 0321, or if dialing from another country, the initial 0 should be omitted after the country code.
International numbering plan
Recommendation E.164 by the International Telecommunication Union establishes an international numbering plan for routing telephone calls between countries. It defines a unique telephone country code with for each member organization, unless they are participating in an integrated numbering plan with other countries. Country codes are dialing prefixes to national telephone numbers and direct call routing to the network of a subordinate numbering plan administration. E.164 permits a maximum length of 15 digits for the complete international phone number consisting of the country code, the national routing code, such as an area code, and the subscriber number. E.164 does not define regional numbering plans; however, it does provide recommendations for new implementations and uniform representation of all telephone numbers.Country codes are necessary only when dialing telephone numbers in countries other than the originating telephone, but many networks permit them for all calls.
Following ITU-T specification E.123, international telephone numbers are commonly indicated in listings by prefixing the country code with a plus sign. This reminds the subscriber to dial the international access code of the country from which the call is placed. For example, the international dialing prefix or access code in all NANP countries is 011, and 00 in most other countries. On modern mobile telephones and many voice over IP services, the plus sign can usually be dialed and functions directly as the international access code. Peer-to-peer SIP uses Dynamic Delegation Discovery System to perform endpoint discovery, and therefore E.164 numbers.