When there's a risk of an area code running out of phone numbers, an 'Overlay Area Code' is created that has the same geographic boundaries as the existing area code. In this case, the 314 code has been Overlayed with the 557 area code. New phone numbers in the Gibson Heights area will be assigned with one of these codes: 314 or 557. As a result, placing a call in the Gibson Heights area will require 10-digit dialing (where you enter both the area code and then the phone number).