Overlay
NANP · also called all-services overlay
Adding a new area code over the same geography as an existing one, so both serve the region and ten-digit dialing becomes required.
An overlay is a form of area-code relief in which a new NPA is assigned to the same geographic area as one or more existing NPAs. Unlike a split, no existing subscriber has to change their number; instead, ten-digit (or 1+ten-digit) dialing becomes mandatory for everyone in the overlay complex.
Overlays are the dominant relief method today because they avoid the disruption of renumbering. A group of area codes sharing one geography is an overlay complex.
An overlay does not change anyone's existing number. It is often confused with a split, which does. The tell: after a split, some people get a new area code; after an overlay, nobody does.
NumberResearch tracks every current relief decision with its dialing dates. See the Relief Tracker ↗.