Area code 636
Area code 636 serves east-central Missouri, mainly the western suburbs of St. Louis. It includes parts of Chesterfield and Fenton in St. Louis County, and all of Washington, Union, De Soto, Troy, St. Charles, Jefferson, and Warren counties, as well as the towns of High Hill and Jonesburg in Montgomery County. It was created in 1999 in a split from area code 314. 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. As a result, St. Louis' outer western suburbs switched to area code 636 on May 22, 1999. A permissive dialing period was allowed in which customers could still complete calls using 314.