Glossary Phone numbers
What is Area code?
Also called npa, numbering plan area, dialing code
An area code is the three-digit prefix that identifies a geographic region within the North American Numbering Plan. In carrier systems it is called the NPA, or Numbering Plan Area. In the number +1 (302) 555-0147, 302 is the area code and covers the state of Delaware.
Overlays, and why you dial ten digits
When a region runs short of numbers, the usual remedy is an overlay: a second area code is assigned to exactly the same territory. Because two codes now serve one area, seven-digit dialling becomes ambiguous and the whole region moves to ten digits. This is why some places have several area codes covering one city and others, like Delaware, still have exactly one.
What an area code signals to a caller
An area code is the fastest signal a business gives about where it is, and it works before anyone answers. A recognisably local number gets picked up more often than an unfamiliar out-of-state one, which is why businesses serving a particular region buy numbers in that region even when nobody sits there. The number does not constrain where the call actually rings.
Area codes are not a location guarantee
Because numbers are portable and calls route over the internet, an area code tells you where a number was originally issued, not where the phone is now. Someone who moved from Chicago to Denver a decade ago may still carry a 312 mobile. This matters in two directions: do not infer a caller's location from their area code for anything that counts, such as emergency routing or tax, and do not assume a local number obliges you to have staff there.