The first time I shipped an SMS signup flow, I assumed a 10-digit number that “looked right” was a number I could text. It is not. A number can pass every format rule in the book and still bounce, ring nowhere, or reject texts outright. The reason traces back to how numbers are laid out in the first place, and in the U.S. and Canada that layout is the North American Numbering Plan (NANP).
If you build anything that validates, routes, or messages phone numbers across the U.S., Canada, and a handful of Caribbean territories, the NANP is the rulebook you are implicitly coding against. Worth knowing where its rules end and where reality takes over.
How a NANP number is laid out
Every NANP number is 10 digits in this shape:
(NPA) NXX-XXXX
- NPA is the Numbering Plan Area, what most people call the area code
- NXX is the central office code, sometimes called the exchange
- XXXX is the subscriber number, the individual line
Take (818) 555-1234. The 818 is the area code (Los Angeles), 555 is the exchange, and 1234 is the line. For international dialing you prepend +1, so the full form is +1-818-555-1234.
That is the whole structure. It is simple, which is exactly why people overtrust it.
Where format checks stop being useful
A number can match the NANP pattern perfectly and still be useless to you. I have watched clean-looking lists fail in production for reasons a regex will never catch:
- Area codes get split and overlaid, so the same code can map to regions you do not expect
- Some prefixes, 555 being the famous one, are reserved or non-routable
- Numbers are ported between carriers, deactivated, and reassigned constantly
- Plenty of valid 10-digit numbers cannot receive SMS at all
A format check answers one question: does this string follow NANP rules? It says nothing about whether the line is active, who carries it, or whether a text will land. Those are the questions that actually cost you money when you get them wrong.
How CheckThatPhone validates NANP numbers
CheckThatPhone is built specifically for U.S. and Canadian numbers inside the NANP. Instead of checking shape, it queries live telecom data and returns:
- Active, routable status
- Line type (mobile, VoIP, landline)
- Portability and current carrier details
- Deliverability and present condition
- Geographic and timezone information
The full set of response fields is in the API documentation.
Where this fits in B2B messaging
The places I have seen this pay off are the obvious ones, and they add up fast:
- Qualifying and filtering sales leads
- Cleaning SMS campaign lists before you pay to send
- Routing customer support calls correctly
- Catching fraud at signup
- Validating numbers on the form, before the bad data lands in your CRM
Next step
The NANP gives you a reliable structure to parse against, and that is genuinely useful. It just does not tell you whether a number is alive. Pair the format rules with a real-time check and you stop guessing about deliverability.
Read the CheckThatPhone documentation to see how NANP numbers get validated in practice, or explore available pricing plans to start validating numbers in your pipeline.