Phone numbers began in 1879 when callers stopped asking the operator by name
Early telephone users simply told the switchboard operator whom they wanted. In 1879, in Lowell, Massachusetts, numbers replaced names. Those first numbers ran just one to three digits and were spoken aloud; today's international format allows up to fifteen, and many are assigned to gadgets that cannot even make calls.
A telephone number is the address of an endpoint on a network. When dialled, its digits travel to an exchange, which routes the call to a local subscriber or onward through the public network, a scheme called destination code routing. Operators hand out numbers within national or regional numbering plans. As networks merged into a worldwide system, numbers lengthened, and until the 1960s many included letters standing for exchange names.
Plans come in two broad styles. Closed plans, like the North American Numbering Plan, give every number the same length: three digits for the area, three for the local exchange office, then four identifying the line, which allows seven-digit local dialling where no overlapping area codes exist. European countries more often built open plans with numbers of varying length. Everywhere, short codes cut through the complexity; emergency services answer to 911, 999, 112, 000 and others, depending on the country.
Internationally, ITU-T recommendation E.164 caps a full number at 15 digits, starting with a country code, then usually an area or service code and the subscriber number. A companion standard, E.123, says printed numbers should open with a plus sign and the country code. Landline callers swap the plus for their own country's international prefix, while many mobiles let users hold down the zero key on GSM handsets, or sometimes the star key on CDMA ones, to enter it directly. Mobile network standards store numbers in a ten-byte binary-coded decimal field, with extension blocks for anything beyond 20 digits.
Some ranges serve special roles. Radio stations running phone-in contests get high-capacity lines sharing one prefix in each metro area, such as 404-741 in Atlanta and 305-550 in Miami, with final digits echoing the station's frequency or name. Numbers once reached modems, pagers and fax machines, and now they identify tablets, televisions, game controllers and mobile hotspots that only ever move data.
Source: Telephone number
