Area code 636 serves east central Missouri near (but not including) St. Louis, including parts of the city of Chesterfield, and all of Union, De Soto, Troy, St. Charles County, Jefferson County, and Warren County.
Area code 636 serves east central Missouri near the St. Louis area, though it doesn't include the city of St. Louis or the majority of St. Louis County. It includes parts of the city of Chesterfield, and all of Union, De Soto, Troy, St. Charles County, Jefferson County, and Warren County.