A local number in the NANP is a geographic subscriber number: a 10-digit NPA-NXX-XXXX assigned to a rate center. NPA is the Numbering Plan Area (area code). NXX is the central-office / exchange code (the next three digits). XXXX is the line number. In E.164 the same line is +1 plus those ten digits. One NXX contains up to 10,000 line numbers (0000-9999), subject to reserved and special values.
The rate center is the geographic unit LECs use to decide what is a local call vs a toll call, and it is the unit that appears in the LERG (Local Exchange Routing Guide) for that NPA-NXX. Two numbers in the same city can sit in different rate centers. Two numbers that look “local” on a map can still be rated as toll if they are not in the same local calling area. Number pooling later split NXX blocks into thousands-blocks, but the rate-center association remains.
Geographic vs non-geographic
| Number type | Example pattern | Tied to a rate center? | How it ports / moves |
|---|---|---|---|
| Local (geographic) DID | NPA-NXX-XXXX in a listed rate center | Yes | NPAC LRN Number porting |
| Toll-free 8XX | 800/888/877/866/855/844/833-NXX-XXXX | No | RespOrg change in SMS/800 |
| N11 / service codes | 211, 311, 411, 511, 611, 711, 811, 911 | Service, not a subscriber line | Not portable subscriber numbers. E911 is its own routing system. |
| Other non-geo NANP | 5XX personal communications, 900 premium, some 6YY | No (or special) | Plan-specific; not ordinary local DIDs |
Overlays and 10-digit dialing
When an area code exhausts, NANPA may add an overlay NPA on the same geography. Overlay regions require 10-digit dialing even for calls across the street. Seven-digit local dialing is largely gone in overlay NPAs. A local DID in an overlay still has one home NPA-NXX and one rate center; the overlay only adds more area codes on the same land.
Identity, emergency, and messaging
- Caller ID from a local number presents a geographic NPA, which callers often treat as “nearby.” That is a display effect, not a proof of physical location.
- E911 routing for a VoIP local number depends on the registered service address, not on the NPA-NXX’s historic central-office building.
- Number porting keeps the digits and the rate-center identity while changing the serving carrier (LRN).
- A2P text from a local 10-digit number in the US is 10DLC, which is a messaging registration regime, not a numbering plan.