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This rounded the total number of digits in a subscriber telephone number to ten: a three-digit area code, a three-digit central office code, and four digits for each line. This fixed format defined the North American Numbering Plan as a closed numbering plan , [ 27 ] as opposed to developments in other countries where the number of digits was ...
Interchangeable numbering plan area (NPA) codes and central office codes are codes that permit any of the ten numerals in the middle position of the area code and the central office code, which both are three-digit numbers. [1] [2] This change in numbering format was implemented first for central office codes by 1973, which eliminated the ...
Telephone dial number card of c.1948 with the local telephone number 4-5876 in Atlantic City, NJ, using the central office prefix 4, later converted to AT4 Face of a 1939 rotary telephone dial with the telephone number LA-2697, which includes the first two letters of Lakewood, New Jersey, as the central office prefix, later converted to LA6.
As phone lines became more popular—between 1942 and 1962, the number of phones in the U.S. grew 230% to 76 million—telephone companies realized they would run out of phone numbers.
National telephone numbers are defined by national or regional numbering plans, such as the European Telephony Numbering Space, the North American Numbering Plan (NANP), or the UK number plan. Within a national numbering plan, a complete destination telephone number is typically composed of an area code and a subscriber telephone number.
Telephone numbers listed in 1920 in New York City having three-letter exchange prefixes. In the United States, the most-populous cities, such as New York City, Philadelphia, Boston, and Chicago, initially implemented dial service with telephone numbers consisting of three letters and four digits (3L-4N) according to a system developed by W. G. Blauvelt of AT&T in 1917. [1]
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).
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. [2] Over the course of telephone history, telephone numbers had various lengths and formats and even included most letters of the alphabet in leading positions when telephone ...