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Recommendation E.123 specifies the format of telephone numbers assigned to telephones and similar communication endpoints in national telephone numbering plans. In examples, a numeric digit is used only if the digit is the same in every number, and letters to illustrate groups.
Precedence is still a feature of military telephone networks, but using number combinations. For example, entering 93 before a number is a priority call. Present-day uses of the signals A, B, C and D are rare in telephone networks, and are exclusive to network control. For example, A is used in some networks for cycling through a list of carriers.
Microsoft canonical address format for telephone numbers [2] [3] derives from E.123 international notation by allowing explicit indication of area code with parentheses. The canonical format is used by the Telephony API (TAPI) , a Windows programming interface for dial-up fax, modem, and telephone equipment.
In December 1930, New York City became the first city in the United States to adopt the two-letter and five-number format (2L-5N), which became the standard after World War II, when the Bell System administration designed the North American Numbering Plan to prepare the United States and Canada for Direct Distance Dialing (DDD), and began to ...
These letter assignments have been used for multiple purposes. Originally, they referred to the leading letters of telephone exchange names. In the mid-20th century United States, before the switch to All-Number Calling, telephone numbers had seven digits, including a two-digit prefix which was expressed in letters rather than digits, e.g.; KL5 ...
Precedence is still a feature of military telephone networks, but using number combinations. For example, entering 93 before a number is a priority call. Present-day uses of the signals A, B, C and D are rare in telephone networks, and are exclusive to network control. For example, A is used in some networks for cycling through a list of carriers.
An automatic number announcement circuit (ANAC) is a component of a central office of a telephone company that provides a service to installation and service technicians to determine the telephone number of a telephone line. The facility has a telephone number that may be called to listen to an automatic announcement that includes the caller's ...
In NANP telephone number specifications, the letter N represents a numeral from 2 through 9, while the letter X represents any numeral. Thus, NXX is a number from 200 through 999, while XXXX is a range from 0000 through 9999. The first three digits of a telephone number are the numbering plan area code (NPA code, or simply NPA).