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Area code 412 is a Commonwealth of Pennsylvania telephone area code which serves Pittsburgh, most of surrounding Allegheny County (except the northern edge of Allegheny County served by Consolidated Communications, formerly the North Pittsburgh Telephone Company) and small portions of Washington and Westmoreland counties.
On February 1, 1998, nearly all of the old 412 territory outside of Allegheny County was split off into area code 724, which completely surrounds 412. Bell Atlantic, the main telephone provider in Pennsylvania, preferred to implement it as an overlay in order to spare residents the burden of changing their numbers. However, overlays were still a new concept at the time, and met with some resistance due to the need for ten-digit dialing. For this reason, 724 was implemented as a split, making it one of the six pairs of "doughnut area codes" in the numbering plan.
This was intended as a long-term solution, but within two years both 412 and 724 were close to exhaustion once again due to the proliferation of cell phones and pagers. By this time, overlays had gained more acceptance, and for this reason area code 878 was overlaid onto both 412 and 724 on August 17, 2001. Although numbers were not assigned in 878 until 2013 (and are only being assigned in the 724 area until 412 runs out of prefixes), ten-digit dialing has been mandatory across southwestern Pennsylvania since 2001.
Pittsburgh, Bethel Park, Penn Hills, Plum, Carnegie, West Mifflin, Fox Chapel, Franklin Park, McCandless, McKeesport, Millvale, Monroeville, Mount Lebanon, Mount Oliver, Oakmont, Robinson Township, Ross Township, Sewickley, Shaler Township, South Fayette, Upper Saint Clair, The Borough West View