Ten-Digit Dialing
NANP · also called 10-digit dialing, 1+10 digit dialing
Dialing the full area code plus number for local calls, required wherever an overlay is in place.
Ten-digit dialing means callers must dial the full ten-digit number (NPA + NXX + line) even for local calls, rather than just seven digits. It becomes mandatory wherever an overlay is introduced, because a seven-digit number is no longer unique when two area codes share the same geography.
The transition to mandatory ten-digit dialing is announced in a planning letter with permissive- and mandatory-dialing dates.
Common misconception.
Ten-digit dialing is a consequence of overlays, not a number change. Your number stays the same; only how you dial local calls changes.