<P> Exchange prefixes were added to small - town numbers to extend four or five - digit local numbers to the standardised seven - digit length, matching in length the then - longest local numbers in the largest major US markets . </P> <P> Within the multinational calling area administered by the North American Numbering Plan, telephone numbers are composed of three fixed - length fields: a three - digit numbering plan area (NPA) code (area code), a three - digit (NXX) central office code, and a four - digit (XXXX) station number . </P> <P> In seven - digit dialing, only the central office code and the station number is dialed, indicating that the call destination is within the local area code . This was the standard in most of North America from the 1950s onward . In some small villages with only one local exchange, it may have been permissible to dial only the four - digit station number . </P> <P> A long distance call within the same area code could often be dialed as 1 + 7D, without using an area code . The scheme relied on the second digit of an area code being 0 - 1 and the second digit of a local exchange being 2 - 9 . This dialing plan was incompatible with the introduction of area code 334 and area code 360, and was therefore eliminated by January 1, 1995 in the United States, and by September 1994 in Canada . It also was eliminated as early as 1981 in some numbering plan areas in the United States that had introduced interchangeable central office codes . </P>

When did phone numbers change to 7 digits