NumberGlossaryCommonly confused terms
Pairs people mix up all the time — side by side, with the tell that separates them.
Overlay vs Split
Original Assignee vs Current Service Provider
NXX (prefix) vs Thousands-Block
Caller ID vs CNAM
Area Code vs Rate Center
Numbering Plan Area (NPA)
The three-digit area code that begins a North American telephone number (the "NPA" in NPA-NXX-XXXX).
An area code identifies a numbering area, not a city. Its boundaries follow rate-center geography, not municipal lines — and because of number portability, a number's area code does not reliably indicate where the caller physically is.
Rate Center
A geographic area used to define local calling boundaries and to which central-office codes are assigned.
A rate center is not a city. Its boundaries are a telephone-network construct for local-calling and number-assignment purposes, and they frequently cross municipal or ZIP-code lines.
How to tell them apart. The tell: an area code is the 3-digit NPA; a rate center is the geographic unit that codes are actually assigned to. Neither is a city.