Illinois currently utilizes 13 area codes. When first established by AT&T and the Bell System in 1947, 217 covered central Illinois, 312 covered the Chicago metropolitan area, 618 covered southern Illinois including East St. Louis and Carbondale, and 815 covered northern Illinois except Chicago and Quad Cities. The most recently added was 872 in 2009 as an overlay to 312 and 773. Today, 815 is the most populated with an estimated 1.8 million of the state's 12.8 million residents.
Sources: North American Numbering Plan Administration (NANPA), telecom providers, and the U.S. Census Bureau.