How a number is built
The United States shares one numbering system, the North American Numbering Plan (NANP), with Canada, US territories and a number of Caribbean countries. Every number in it has ten digits after the country code 1:
| Part | Digits | Pattern | Meaning |
|---|---|---|---|
| Area code (NPA) | 3 | NXX | A geographic region, or a special service such as toll-free |
| Exchange (central office code) | 3 | NXX | A block of numbers within the area code, originally tied to a local switch |
| Line number | 4 | XXXX | The individual line |
In the pattern, N means any digit from 2 to 9 and X means any digit from 0 to 9. So neither an area code nor an exchange can start with 0 or 1. A number like (123) 456-7890 can't be a real NANP number.
Special codes
N11 codes
Three-digit codes ending in 11 are service codes, not area codes or exchanges. The best known is 911 for emergencies. Others include 211 for community services, 311 for non-emergency city services in many places, 411 for directory assistance, 511 for traffic information, 711 for relay services for people who are deaf or hard of hearing, and 811 to have underground utilities marked before digging. The 988 Suicide and Crisis Lifeline is a separate three-digit code launched in July 2022.
555 numbers
The exchange 555 is mostly unassigned. The range 555-0100 through 555-0199 is set aside for fictional use in films, TV and examples, which is why it appears in this page's checker. 555-1212 has long been used for directory assistance. A caller ID showing another 555 number is a strong sign of a fake.
Toll-free codes
800, 833, 844, 855, 866, 877 and 888 are toll-free: the business pays for the call. They aren't tied to a location, and scammers use them too, so a toll-free number is no proof of legitimacy.
900 numbers
Calls to 900 numbers can carry premium charges. Most phone plans allow you to block them.
Codes that look domestic but aren't
Because Caribbean countries in the NANP use the same ten-digit format, some numbers look like US numbers but are international. Examples include 242 (Bahamas), 246 (Barbados), 284 (British Virgin Islands), 473 (Grenada), 649 (Turks and Caicos), 809, 829 and 849 (Dominican Republic), and 876 and 658 (Jamaica). Calls to them may be billed at international rates. They're common in one-ring scams. US territories such as Puerto Rico (787, 939), the US Virgin Islands (340) and Guam (671) are generally billed like domestic calls on most US plans, but check your own plan.
Overlays and ten-digit dialing
When a region runs out of numbers, a new area code is usually added as an overlay covering the same area. Neighbors can then have different area codes, and everyone has to dial all ten digits for local calls. Ten-digit dialing also became necessary in many more areas around 2021 to make way for 988.
What an area code doesn't tell you
An area code shows where a number was first assigned. People keep their numbers when they move or switch carriers, and internet numbers can be chosen from almost any code. Treat the area code as a hint about history, not a location. For more on what a number does reveal, see how reverse lookup works.