Glossary
What is NPA-NXX?
NPA-NXX is the first six digits of a North American phone number: the three-digit area code (NPA) followed by the three-digit prefix or exchange (NXX). It tells you where a number block was first assigned and to which carrier, but not who holds the number today.
The parts of a North American phone number
| NANP | The North American Numbering Plan: the shared 10-digit numbering system used by the US, Canada and a number of Caribbean countries under country code +1. |
| NPA | Numbering Plan Area, better known as the area code. The first three digits of a 10-digit number, such as 212 or 415. |
| NXX | The central office code, also called the exchange or prefix. The three digits after the area code, such as the 555 in 212-555-0100. |
| Line number | The last four digits. Together with the NPA-NXX they identify one specific phone number. |
| N and X | In the NPA-NXX pattern, N stands for any digit from 2 to 9 and X stands for any digit from 0 to 9. So an area code or prefix never starts with 0 or 1. |
| Thousands block | NPA-NXX-X: a block of 1,000 numbers. In many US areas, numbers are assigned to carriers in thousands blocks rather than whole 10,000-number prefixes. |
How NPA-NXX assignment works
Under the North American Numbering Plan, every 10-digit number follows the pattern NPA-NXX-XXXX. Area codes and central office codes are assigned by the North American Numbering Plan Administrator to a rate center, a defined local calling area, and to a carrier that requested them. In much of the US, numbers are now handed out in thousands blocks (NPA-NXX-X) so that one prefix can be shared by several carriers.
Once a carrier holds a block, it issues individual numbers to subscribers. If a subscriber later switches providers and keeps the number, the move is recorded in number portability data, and the original NPA-NXX assignment no longer reflects who serves that number.
Why NPA-NXX matters if your team calls or texts leads
- Format checks: an area code or prefix starting with 0 or 1, or a prefix in the N11 pattern, is not a valid US number. Formatters and validators use these rules to flag typos before a dial.
- Calling hours: the area code is the usual starting point for a lead's time zone when you have no address, even though people keep numbers when they move.
- Carrier and line type: the prefix only shows the original assignment. For the current carrier and whether a number is mobile, landline or VoIP, use a carrier lookup.
- Not a compliance check: NPA-NXX data says nothing about Do Not Call status or consent.
What the prefix tells you vs. a carrier lookup
| NPA-NXX alone | Carrier lookup | |
|---|---|---|
| Where the block was assigned | Yes. Each NPA-NXX is assigned to a rate center, so the prefix points to an original geographic area | Also returns the current carrier and line type |
| Original carrier | Yes, the carrier the block or thousands block was first assigned to | Returns the carrier that holds the number now |
| Current carrier | Not reliably. A ported number has moved to another carrier | Yes, from carrier data |
| Line type | A guess based on the block | Mobile, landline, VoIP or toll-free |
| Valid or dead | No | TrueSearch returns valid or dead |
| Where the person lives now | No. People keep their numbers when they move | No. A lookup does not return a location |
What TrueSearch returns, and does not
| Valid or dead (disconnected / not assigned) | Yes, every number |
| Line type: mobile, landline, VoIP, toll-free | Yes, every number |
| Carrier name | Yes, every number |
| Bulk upload (CSV or Excel) | Yes, results in about 30 seconds per 1,000 numbers |
| REST API | Yes (JSON, one API key) |
| Claude / ChatGPT / Cursor connector (MCP) | Yes |
| National or state Do Not Call check | No, not yet |
| Known TCPA litigator check | No, not yet |
| FCC Reassigned Numbers Database check | No |
| Starting price | $97/month for 5,000 numbers, 7-day free trial |
| One-off list | $20 per 1,000 numbers, no subscription |
Upload a CSV or Excel file. Every number comes back valid or dead, mobile, landline, VoIP or toll-free, with its current carrier.
Starter: $97/month for 5,000 numbers after the 7-day trial. Cancel anytime.
Questions
What does NPA-NXX mean?
NPA-NXX is the first six digits of a North American phone number. NPA (Numbering Plan Area) is the three-digit area code and NXX is the three-digit central office code, also called the exchange or prefix. In 212-555-0100, the NPA is 212 and the NXX is 555.
What do the letters N and X stand for?
They describe which digits are allowed. N means any digit from 2 to 9, and X means any digit from 0 to 9. That is why no US area code or prefix begins with 0 or 1: 0 reaches an operator and 1 is used as a prefix for long-distance dialing.
Can a prefix be 911 or 411?
No. Codes in the N11 pattern (211, 311, 411, 511, 611, 711, 811 and 911) are reserved as service codes, so they are not used as central office codes.
Is 555 a real prefix?
Partly. The numbers 555-0100 through 555-0199 are reserved for fictional use in film, TV and advertising, which is why so many on-screen numbers use them. Other 555 numbers can be assigned, and 555-1212 has long been used for directory assistance.
Does the NPA-NXX tell me the carrier?
Only the carrier the block was originally assigned to. Because of number portability, a subscriber can keep their number when switching providers, so the current carrier can be different. To know who holds the number now, you need a carrier lookup that uses carrier data rather than the prefix.
Does the NPA-NXX tell me where the person lives?
It tells you the rate center the block was assigned to, which is usually where the number was first issued. People keep numbers when they move, so it is not a reliable guide to where they live today. It is a reasonable starting point for time zone and calling hours when you have nothing better.
Who assigns area codes and prefixes?
The North American Numbering Plan Administrator (NANPA) assigns area codes and central office codes under FCC oversight in the US. Carriers request codes or thousands blocks for a rate center and then issue individual numbers to subscribers.
Does TrueSearch check Do Not Call status?
No. TrueSearch checks validity, line type and carrier only. For Do Not Call status, use the National Do Not Call Registry at donotcall.gov.
Related: free phone number prefix lookup, free carrier lookup, what is a carrier lookup?, what is a ported number?, what is E.164 format? and the calling hours checker.
By industry
By carrier
Compare
This guide is general information, not legal advice. TrueSearch for Business checks validity, line type and carrier from licensed telecom data. It does not check the National Do Not Call Registry, state DNC lists or the FCC Reassigned Numbers Database. Not a consumer reporting agency; not for FCRA purposes.