Telephone Number

A telephone number is a set of digits assigned to a fixed-line telephone subscriber station connected to a phone line, a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or other devices for data transmission via the public switched telephone network (PSTN) or other public and private networks.

Telecom switchboard and operator

A telephone number is used as an address when employing a destination code routing scheme to transfer calls. A calling party enters or dials a telephone number on the originating telephone set, which transmits the sequence of digits to a telephone exchange as part of the signaling process. The exchange connects the call to another locally connected subscriber or to the called party through the PSTN. Telephone numbers are allocated to customers by telephone service providers, which can be commercial companies, state-controlled administrations, or other telecommunication sector groups, as part of a national or regional telephone numbering scheme.

Telephone numbers were first used in Lowell, Massachusetts, in 1879, when callers connecting to the switchboard operator were asked for subscriber names. Telephone numbers have varied in length and structure throughout history, and even featured most letters of the alphabet in leading places when telephone exchange names were often used until the 1960s.

To activate specific telephone service features, telephone numbers are frequently dialed in combination with additional signaling code sequences, such as vertical service codes.

Telephone numbering plan

A telephone numbering plan is a sort of numbering scheme used in telecommunications to allocate phone numbers to subscriber phones and other telephony endpoints.

Geographic location often plays a factor in the sequence of numbers issued to each telephone user in public numbering systems. Many numbering plan administrators split their service territory into geographic zones identified by a prefix, also known as an area code or city code, which is a group of numbers that comprise the most important portion of the dialing sequence to contact a telephone subscriber.

Telephone numbering plan

Various design methodologies for numbering systems have emerged from the historical growth of various telephone networks and local requirements. A frequent distinction is made between a closed numbering plan, such as that used in North America, which uses fixed-length area codes and local numbers, and an open numbering plan, which uses variable-length area codes, local numbers, or both when assigning a telephone number to a subscriber line. The latter was mostly developed in Europe.

The International Telecommunication Union (ITU) has devised an E.164 comprehensive numbering strategy for consistent interoperability of its member state or regional administrations' networks. It is, however, an open numbering strategy, with telephone numbers having a maximum length of 15 digits. For international destination routing, the standard specifies a country calling code (country code) for each state or area, which is prefixed to each national numbering plan telephone number.

In a business or organizational campus, private numbering plans occur in telephone networks that are privately operated. A private branch exchange (PBX), which offers a central access point to the PSTN and also manages internal conversations between telephone extensions, may be used to enable such systems.

Area Code

Telephone administrations that oversee large-scale telecommunication infrastructure, such as a country, frequently split the region into geographic divisions. This promotes autonomous control by administrative or historical subdivisions of the area or country, such as states and provinces. A route code is assigned to each subdivision area in the numbering plan. This idea was initially created in the 1940s when the Bell System in the United States was preparing a countrywide numbering scheme for Operator Toll Dialing and direct distance dialing (DDD), a system that culminated in the North American Numbering Plan for World Zone 1. AT&T separated the United States and Canada into numbering plan areas (NPAs) and issued each one a three-digit prefix, the numbering plan area code, which became known as NPA code or simply area code in short form. Each telephone number issued in its service area has an area code prefixed to it.

Other national telecommunications bodies utilize a variety of area code forms and dialing procedures. Area code prefixes can be either fixed or variable in size. In the NANP, area codes contain three digits, although in Brazil, two digits are used, and in Australia and New Zealand, one digit is used. Argentina, Austria (1 to 4), Germany (2 to 5 digits), Japan (1 to 5), Mexico (2 or 3 digits), Peru (1 or 2), Syria (1 or 2), and the United Kingdom all use variable-length forms. Aside from digit count, the format might be limited to specific digit patterns. To minimize confusion and misdialing, the NANP placed particular constraints on the range of numbers for the three slots at times, and required assignment to geographical areas avoiding adjoining areas having identical area codes.

Country Code

Country codes are only required when calling numbers in countries different than the one from which the call originated. These numbers are called before the national number. International phone numbers are displayed in listings by prefixing the country code with a plus sign (+), as is customary. The subscriber is reminded to dial the international calling prefix for the country from which the call is made. For example, in all NANP nations, the international dialing prefix or access code is 011, although in most European countries, it is 00. In some GSM networks, it may be feasible to dial + instead of the international access code, which the network provider may detect immediately.

International Dialing Prefix

A trunk prefix used to choose an international telephone circuit for establishing an international call is known as an international call prefix or dial out code. It's currently known as an IDD prefix (international direct dialing), and each country has its own NDD prefix (national direct dialing). Before dialing the country calling code and the destination phone number, you must first dial the international dialing prefix. It's the same thing as an international access code or an exit code. For calls to another country, a country's telephone numbering plan includes an international call prefix.

Most nations, but not all, have adopted the sequence 00 as a standard for an international call prefix, as recommended by the International Telecommunication Union (ITU). The 00 prefix is used by some nations, followed by the international carrier code.