La mayoría de los países utilizan números de longitud fija (para las líneas normales al menos), de forma que los equipos terminales pueden determinar la longitud necesaria del número de teléfono que se está marcando. También es posible que cada abonado tenga un conjunto de números más cortos para los destinos más frecuentes. Estos números cortos son traducidos automáticamente a números de teléfono únicos antes de establecer la llamada. Algunos servicios especiales pueden tener números cortos; es el caso de algunos números de urgencias como el 112 o 911 y otros que dependen del país o la región. Muchos sistemas también permiten las llamadas dentro de un área local sin marcar el prefijo local.

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A telephone number is a sequence of digits assigned to a fixed-line telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices for data transmission via the public switched telephone network (PSTN) or other private networks. Most telephone numbers are assigned to one telephone line or one mobile telephone, and most lines or mobiles have one number. A telephone number serves as an address for switching telephone calls using a system of destination routing. Telephone numbers are entered or dialed by a calling party on the originating telephone set, which transmits the sequence of digits in the process of signaling to a telephone exchange. The exchange completes the call either to another locally connected subscriber or via the PSTN to the called party. The use of telephone numbers instead of subscriber names to indicate to the switchboard operator the call destination was developed and first used in the autumn of 1879 in Lowell, Massachusetts during a measles epidemic. A local physician, Moses Greeley Parker, realized that if all four of the city's operators were incapacitated by the epidemic, their replacements would have great trouble quickly learning the assignment of the 200 jacks on the switchboard to subscribers. He recommended the use of numbers instead. "The local Bell company management at first protested that its customers would consider their designation by numbers to be beneath their dignity; nevertheless, it saw the logic of the suggestion and followed it. The subscribers were not outraged; the epidemic quickly passed, but telephone numbers did not."

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