How NANP area codes work, why one city can have several, and how to find where a phone number is from.
The North American Numbering Plan divides the US, Canada and much of the Caribbean into three-digit area codes. The format is NXX-NXX-XXXX: area code, central-office code, line number.
When a region runs out of numbers, regulators either split it (the old code keeps part of the region) or overlay it (a new code covers the same area and everyone dials ten digits). Overlays are why one city may have several area codes — New York, Los Angeles and Houston all have several.
The free Area Codes directory covers NANP area codes and world dialing prefixes, searchable by code, city, state or country, with the places each code covers.
Number portability means a phone's area code no longer reliably indicates where its owner lives — it shows where the number was first assigned. For landlines it is still a good guide.