Area code 636 serves east-central Missouri, mainly the metro area of St. Louis excluding the city of St. Louis and most of St. Louis County. Area code 636 includes parts of the community of Chesterfield, Fenton, and all of Washington, Union, De Soto, Troy, St. Charles, Jefferson, and Warren counties. Area code 314, now assigned to St. Louis and some suburbs, formerly extended to Washington and St. Francois counties.
Before 1996, 314 covered the entire eastern half of Missouri. In 1996, most of the old 314 territory outside of the St. Louis area became area code 573. This was intended as a long-term solution, but within two years 314 was almost out of numbers again due to the proliferation of cell phones, pagers and fax machines. The supply of numbers was further limited because the St. Louis LATA spills into much of southern Illinois, meaning several numbers that were part of area code 618 across the border in Illinois weren't available. As a result, most of St. Louis' outer-ring suburbs were switched to area code 636 on May 22, 1999. A permissive dialing period was allowed in which customers could still complete calls using 314.