Telephone numbering plan
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.
Numbering plans may follow a variety of design strategies which have often arisen from the historical evolution of individual telephone networks and local requirements.
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.
The standard defines a country code for each member region which is prefixed to each national telephone number for international destination routing.
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.
[3] 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 individual telephone exchanges (central office code), such as a wire centers.
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.
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.
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.
A caller from within the network only dials the extension number assigned to another internal destination telephone.
The internal number assignments may be independent of any direct inward dialing (DID) services provided by external telecommunication vendors.
For numbers without DID access, the internal switch relays externally originated calls via an operator, an automated attendant or an electronic interactive voice response system.
For example, individual extensions at Universität des Saarlandes can be dialed directly from outside via their four-digit internal extension +49-681-302-xxxx, whereas the university's official main number is +49-681-302-0[8] (49 is the country code for Germany, 681 is the area code for Saarbrücken, 302 the prefix for the university).
A common trunk prefix for an outside line on North American systems is the digit 9, followed by the outside destination number.
During signaling, it is common that additional information is passed between switching systems that is not represented in telephone numbers, which serve only as network addresses of endpoints.
This may include the dialing of additional prefixes necessary for administrative or technical reasons, or it may permit short code sequences for convenience or speed of service, such as in cases of emergency.
Despite the closed numbering plan in the NANP, different dialing procedures exist in many of the territories for local and long-distance telephone calls.
To call a number in Sydney, Australia, for example: The plus character (+) in the markup signifies that the following digits are the country code, in this case 61.
In the United States, most carriers require the caller to dial 011 before the destination country code.
Many mobile handsets automatically add the area code of the set's telephone number for outbound calls, if not dialed by the user.
1+ dialing to any area code by an employee can be done quickly, with all exceptions processed by the private branch exchange and passed onto the public switched telephone network.
For example, to call someone in Oslo in Norway before 1992, it was necessary to dial: After 1992, this changed to a closed eight-digit numbering plan, e.g.: However, in other countries, such as France, Belgium, Japan, Switzerland, South Africa and some parts of North America, the trunk code is retained for domestic calls, whether local or national, e.g., While some, like Italy, require the initial zero to be dialed, even for calls from outside the country, e.g., While dialing of full national numbers takes longer than a local number without the area code, the increased use of phones that can store numbers means that this is of decreasing importance.
