How US area codes work
The North American Numbering Plan assigns 379 codes across the United States, 372 of them tied to a geography. What follows explains the parts of the system people most often get caught out by — written once here, rather than repeated onto every code’s page.
What are the three parts of a US phone number?
A ten-digit US number splits into three. The first three digits are the area code, or NPA, which identifies a numbering-plan area. The next three are the exchange, or NXX, which historically identified a rate centre — the switching area a line was served from. The last four identify the line itself. So the area code narrows a number to a region, and the exchange used to narrow it to a town.
What is an overlay, and why does it mean ten-digit dialling?
When a region runs out of exchanges to hand out, the regulator has two options: split the area in two and make half the residents change their number, or lay a second code over the top and issue new numbers from it. The second is an overlay, and it is now the normal choice because nobody has to renumber.
The consequence is that two neighbours on the same street can hold numbers in different area codes. Seven digits are therefore no longer unique inside that region, so local calls have to be dialled as all ten. It also means the area code tells you nothing about which part of town a line is in — the overlaid codes cover exactly the same ground.
The consequence is that two neighbours on the same street can hold numbers in different area codes. Seven digits are therefore no longer unique inside that region, so local calls have to be dialled as all ten. It also means the area code tells you nothing about which part of town a line is in — the overlaid codes cover exactly the same ground.
Does an area code prove where a caller is?
No, in both directions. Number portability lets a person keep their number when they move, so somebody may be carrying a code from a home they left years ago. Internet-based phone services go further and will sell a number for almost any region to a customer anywhere. A number records where a line was first issued, not where it is answered today.
Can you tell whether a number is a mobile or a landline?
Not reliably, and be wary of any site that claims otherwise. Blocks of numbers were once assigned to a single carrier, so the exchange hinted at the line type — but portability broke that link, and a number can now move between carriers and between mobile and landline service. Codes.zone does not publish prefix-level routing data: the source for it is proprietary, and we will not estimate it.
Can you choose which area code a new number gets?
Rarely, and not reliably. Carriers assign from whatever inventory the local rate centre still holds, and in a region with more than one code the older ones are usually exhausted — which is why the newer code exists at all. Some providers will honour a preference if they happen to have spare numbers; most will not offer the choice. A number ported from an old line keeps its original code regardless.
Is calling another area code long distance?
That depends on your plan, not on the area code. Practically every US mobile and landline plan now includes unlimited domestic calling, so a call across the country costs the same as one next door. If you are on an older metered plan, or calling from outside the US, check with your carrier — the charge comes from the plan, and no area code carries a surcharge of its own.
Why do calls appear to come from my own area code?
Because caller ID is set by whoever places the call and is not verified by the network. That is what makes neighbour spoofing work: an operation anywhere in the world can display a number matching the area code — and often the exchange — of the person being called, so the call looks local and gets answered.
What actually stops unwanted calls?
Blocking the individual number rarely helps for long, since spoofed calls change number constantly. What does help: turn on your carrier's call-screening service, which most US carriers now provide free and which uses the STIR/SHAKEN caller-ID authentication framework to flag calls whose origin cannot be verified; switch on your phone's own "silence unknown callers" setting; and register on the National Do Not Call Registry, which will not stop outright criminals but does remove you from legitimate telemarketing lists. Report the rest to the FCC.
If a call you did not expect asks for payment or personal details, hang up and call the organisation back on a number you looked up yourself. Never confirm card numbers or one-time codes to someone who called you.
If a call you did not expect asks for payment or personal details, hang up and call the organisation back on a number you looked up yourself. Never confirm card numbers or one-time codes to someone who called you.