Area code 878 is an overlay area code in Southwestern Pennsylvania, centered in Pittsburgh. It simultaneously overlays the region's two existing area codes, 412 and 724, making ten-digit dialing mandatory in the Pittsburgh area. On September 15, 2015, the first prefix (999) was assigned into the 412 territory. As of February 2020, Verizon is issuing 878 telephone numbers inside 412's original territory.