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Glossary · Numbering

What is NPA-NXX?

  • Also called: npa nxx, exchange code, central office code

Reviewed by Sujan ThapaliyaLast updated

Definition

NPA-NXX is the first six digits of a North American number: the three-digit area code (NPA) plus the three-digit exchange (NXX). Together they identify the switch a number was originally assigned to, which is how numbers are routed and rated.
The three STIR/SHAKEN attestation levels, which underpin how npa-nxx behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, npa-nxx included.

In practice

How npa-nxx actually works

Number portability broke the neat version of this. A ported number keeps its NPA-NXX while actually living on a different carrier's network, so the prefix identifies where a number came from rather than who serves it today.

It is also the unit that neighbour spoofing exploits: matching the callee's NPA-NXX makes a call appear to come from a handful of houses away, which is why carriers now score that pattern aggressively.

For rating and routing purposes NPA-NXX remains the working key, which is why carrier rate tables are still organised around it.

NPA-NXX is also the unit rate tables are built on, which is why international and domestic pricing can look inconsistent. Two numbers in the same city can rate differently if they were issued from blocks assigned to different rate centres, and a carrier quoting a single national rate is averaging that variation rather than removing it.

Worked example

In +1 (415) 555-0123, the NPA is 415 and the NXX is 555 — together the block the number was issued from.

NPA-NXX: common questions

What does NXX mean?
The three-digit exchange code following the area code. It originally identified the local switch serving that block of numbers.
Can I tell which carrier serves a number from NPA-NXX?
Only which carrier it was issued to. Portability means the number may have moved, so a current carrier lookup is required rather than an inference from the prefix.
Why do dialers care about NPA-NXX?
Because matching it to the person being called is what neighbour spoofing does, and because carrier analytics treat that matching as a signal. Legitimate local presence matches the area code only.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See npa-nxx in the product

Local presence dialing presents a caller ID in the same area code as the person you are calling, because a familiar area code is answered more often than an unfamiliar one. Done properly it uses numbers you actually own and can receive callbacks on; done badly it is number spoofing with a marketing name.

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