When the
American Telephone and Telegraph Company (AT&T) created the first nationwide
telephone numbering plan for the continental United States and Canada in 1947, Pennsylvania was divided into four numbering plan areas. Area code 412 was assigned to the southwestern part of the state, from Butler County to the West Virginia border. On February 1, 1998, most of southwestern Pennsylvania outside of Allegheny County was split into a new numbering plan area with
area code 724.
Bell Atlantic, the main telephone provider at the time in Pennsylvania, had preferred to implement 724 as an
overlay to spare residents and businesses the burden of changing telephone numbers. However, overlays were still a new concept at the time, and met with resistance because of the need for
ten-digit dialing. As a result, 724 was implemented as an
area code split, making 412 one of six pairs of
doughnut area codes in North America. This configuration was intended as a long-term solution, but within two years both 412 and 724 experienced high demand for telephone services from the proliferation of cell phones and pagers, so that further relief became necessary. By this time, overlays had gained more acceptance, so
area code 878 was implemented as an overlay for both 412 and 724 on August 17, 2001. Although telephone numbers were not assigned for 878 until 2013 (and were only assigned in the 724 area until 2015), ten-digit dialing has been mandatory across southwestern Pennsylvania since 2001. ==Service area==