Missouri currently utilizes 6 area codes. When first established by AT&T and the Bell System in 1947, 314 covered eastern Missouri including St. Louis and 816 covered northwestern Missouri including Kansas City. The most recently added was 636 in 1999. Today, 816 is the most populated with an estimated 1.3 million of the state's 6 million residents.
Sources: North American Numbering Plan Administration (NANPA), telecom providers, and the U.S. Census Bureau.