Area codes 314 and 557 serve St. Louis and most of its inner-ring suburbs in neighboring St. Louis County. The area is completely surrounded to the west by area code 636, which serves St. Louis' outer suburbs to the west, south, and north. Across the Mississippi River to the east, 314 borders area code 618, which serves southern Illinois and most of Metro East. 557 was implemented on August 12, 2022 as an overlay to area code 314.
Area code 314 was one of the original area codes created in October 1947. It originally covered most of the eastern third of Missouri, from the Illinois border to Jefferson City. It stretched along nearly the entire width of the state along the Mississippi River. It was split on January 7, 1996, when most of the western, northern, and southern portions were assigned to area code 573, leaving 314 to cover most of the Missouri side of the St. Louis metropolitan area.
The creation of 573 was intended to be a long-term solution. Within only two years, however, 314 was close to exhaustion once again due to the proliferation of cell phones, fax machines and pagers in the St. Louis core. As a result, 314 was reduced to its current size on May 22, 1999, when area code 636 was assigned to St. Louis' outer suburbs and the nearby rural areas. Before the introduction of number portability, 314 was used for all cell phones on the Missouri side of the St. Louis area.
In 2000, the Missouri Public Service Commission announced plans to add an overlay to area code 314, but postponed the implementation date (May 5, 2002) indefinitely, when the commission determined no immediate need for more phone numbers. Conservation measures such as number pooling staved off the immediate need for an overlay. More than 20 years after that announcement, area code 557 was overlaid on August 12, 2022, as the available numbers in 314 neared exhaustion.
Prior to October 2021, area code 314 had telephone numbers assigned for the central office code 988. In 2020, 988 was designated nationwide as a dialing code for the National Suicide Prevention Lifeline, which created a conflict for exchanges that permit seven-digit dialing. This area code was therefore scheduled to transition to ten-digit dialing by October 24, 2021.