AREA CODE OVERLAY
What is an area code overlay? In an overlay, two or more area codes have the same geographic boundary. Existing phone numbers and local calling scopes don't change, but 10-digit local dialing becomes necessary.
Ten digit dialing (area code + seven digit number) is always necessary when two or more area codes cover the same geographic area. In this situation two or more different homes in the same geographic area can have the same seven digit phone number but each would have a different area code. Ten digit dialing within area code overlays permits the phone companies to route your call to the correct subscriber.
Most area code overlays involve two area codes covering the same geographic area, however, three or more area codes are used in some highly populated areas. An example is in the state of New York where area codes (212, 332, 646 and 917) and (718, 347, 917 and 929) are utilized as overlays.