<P> Originally, telephone exchanges consisted of manual boards operated by switchboard operators . Numbers were typically four digits or fewer for local calls within an exchange due to practical limitations (if each line had a jack on the switchboard, four digits or 10000 possible numbers filled a 100 x 100 board). As the number of subscribers grew, multiple exchanges served individual neighborhoods of large cities . A city telephone number consisted of an exchange name and four digits, such as "Pennsylvania 5000". A rural telephone number, often party line, was often up to four digits plus a letter or letter and digits to indicate which of the multiple parties on the line was desired . </P> <P> Various schemes were used to convert these to dialable numbers as dial replaced manual switchboards; many moderately - large cities used a 2L + 4N format where "ADelaide 1234" would be dialled as AD - 1234 (23 - 1234, a six - digit local call). A few of the largest cities, such as New York, used seven dial pulls ("PENnsylvania 5000" became PEN - 5000 and later PEnnsylvania 6 - 5000, dialled PE6 - 5000 or 736 - 5000). </P> <P> The initial 86 area codes were assigned in 1947 as routing codes for operator calls; the first cross-country Bell System direct distance dial call was made in 1951 . The system was based on fixed - length numbers; a direct - dial long distance call consisted of a three - digit area code and a seven - digit local number . Numbers in 2L + 4N cities (such as Montréal and Toronto) were systematically lengthened to seven digits in the 1950's, a few exchanges at a time, so that all local numbers were seven digits when direct distance dialling finally came to town . </P> <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>

When did the 7 digit phone number start