A telephone number serves as an address for switching telephone calls using a system of destination code routing. Telephone numbers are entered or dialed by a party on the originating telephone set. The exchange completes the call either to another locally connected subscriber or via the PSTN to the called party, telephone numbers were first used in 1879 in Lowell, Massachusetts when they replaced the request for subscriber names by callers connecting to the switchboard operator. Telephone numbers are dialed in conjunction with other signaling code sequences, such as vertical service codes. When telephone numbers were first used they were short, from one to three digits, and were communicated orally to a switchboard operator when initiating a call. As telephone systems have grown and interconnected to encompass worldwide communication, in addition to telephones, they have been used to access other devices, such as computer modems, pagers, and fax machines. The number contains the necessary to identify uniquely the intended endpoint for the telephone call. Each such endpoint must have a number within the public switched telephone network. Most countries use fixed length numbers and therefore the number of endpoints determines the length of the telephone number. It is also possible for each subscriber to have a set of numbers for the endpoints most often used. These shorthand or speed calling numbers are translated to unique telephone numbers before the call can be connected. Some special services have their own short numbers The dialing plan in some areas permits dialing numbers in the local calling area without using area code or city code prefixes. For example, a number in North America consists of a three-digit area code, a three-digit central office code. If the area has no area code overlays, seven-digit dialing may be permissible for calls within the area, for each large metro area, all of these lines will share the same prefix, the last digits typically corresponding to the stations frequency, callsign, or moniker. In the international network, the format of telephone numbers is standardized by ITU-T recommendation E.164. This code specifies that the number should be 15 digits or shorter. For most countries, this is followed by a code or city code and the subscriber number. ITU-T recommendation E.123 describes how to represent an international telephone number in writing or print, starting with a plus sign and the country code
A Swiss rotary telephone dial from the 1970s, showing the telephone's number (94 29 68) along with various local emergency services.
Face of a 1939 rotary dial showing a 2L-4N style alphanumeric telephone number LAkewood-2697.
2008 photo shows a hairdressing shop in Toronto with an exterior sign showing the shop's telephone number in the old two-letters plus five-digits format.
Telephone numbers for sale in Hong Kong. The prices are higher for desirable numbers.