Area codes 815 and 779 are telephone area codes in the North American Numbering Plan (NANP) for most of northern Illinois outside the Quad Cities areas. It also serves the Joliet area and some of Chicago’s western and northwestern suburbs. 815 was one of the four original Illinois area codes set up in 1947. Area code 779 was assigned to the same numbering plan area in 2009 to form an overlay.