How area codes and overlays work
Published
What an area code is, how a new one is added when an old one fills up, and what an overlay means for a town: two or more codes in one place, old numbers kept.
What an area code is
An area code is the first three digits of a ten-digit phone number in the North American Numbering Plan (NANP). The plan is the shared numbering system of the United States and its territories, Canada, Bermuda and many Caribbean countries. In the plan's own terms, an area code is a Numbering Plan Area code, or NPA.
After the area code come three digits called the prefix, or central office code, and then a four-digit line number. The plan dates from 1947, when it was drawn up to make long-distance calls easier to dial directly.
Sources: NANPA: about the North American Numbering Plan; NANPA: area code relief frequently asked questions
Why new area codes are needed
Phone companies get their numbers from the numbering administrator, NANPA, in blocks: a whole prefix of ten thousand numbers, or a block of a thousand where numbers are pooled. Each area code has a fixed set of prefixes. When nearly all of them are handed out, the code is said to reach exhaust, and the region needs another area code.
NANPA tracks how fast each area code is filling. It tells the state regulator and the phone industry when a code is expected to exhaust within three years. The industry then proposes how to add the new code, and the state commission makes the final decision.
Splits: dividing the map
One way to add a code is a geographic split. The old area is divided: one part keeps the existing code and the rest gets a new one. In 1998, for example, North Carolina split 919. The Raleigh area kept 919, and the Rocky Mount area moved to the new 252.
A split leaves one area code in each place, so local calls inside it can stay at seven digits. The cost is that about half of the customers in the old area get a new area code, and every card, sign and ad that shows their number has to change.
Sources: NANPA planning letter PL-NANP-112: split of the 919 area code (North Carolina); NANPA: area code relief frequently asked questions
Overlays: adding a code on top
In an overlay, the new area code covers the same ground as the old one. Everyone keeps the number they already have. New lines can get numbers from the new code, so neighbors, or two phones in the same office, can end up with different area codes.
Because two codes now serve the same place, local calls there have to include the area code. NANPA calls overlays the industry-preferred form of relief, and every new geographic area code put in service in the United States in 2025 was an overlay.
Sources: NANPA: area code relief frequently asked questions; NANPA: 2025 annual report (new area codes, overlays and dialing plans)
What an overlay means for a town
A town inside an overlay has more than one area code, and calls that were local before the new code arrived stay local. West Virginia is one example: its only area codes are 304 and 681, which overlay each other. In New York, 212 shares its area with 646, 332 and 917.
On this site, a town in an overlay lists every code in the group, because a new local number there can come from any of them.
Sources: NANPA: 2025 annual report (new area codes, overlays and dialing plans); NANPA: area code relief frequently asked questions