Area codes are part of the North American Numbering Plan, an integrated telephone numbering plan of 24 countries and territories created and installed by AT&T in 1947. Only long-distance operators first used the three-digit area codes; the first customer-dialed calls using area codes occurred in 1951. Originally, there were 86 area codes, with the areas of greatest population getting the area codes taking the shortest time to dial on rotary telephones. In the early 1990s, the prefix digit 1 was required for making long-distance calls.
The Knoxville area code was originally assigned as 615. The area code for the Knoxville area was changed from 615 to 423, effective September 11, 1995, because of the need to expand the number of area codes. It was changed again in 1999 and became 865. The 865 prefix was chosen for the Knoxville area because the letters associated with those digits on telephone dials spelled Vol.