How Do Area Codes Work?

Area codes are formatted as NXX, where N may be any number between and including 2 and 9, and X may be any number between and including 0 and 9. The NXX format can provide 800 possible area code combinations; some for special purposes, and some to be issued as area codes for regular telephone numbers.

Reserved area codes are:

Easily Recognizable Codes
When the second and the third digits are the same in an area code, it is referred to as an ERC (easily recognizable code). These are reserved for special phone services, such as 800 and 888 which are used for toll-free numbers.

N11
The 8 ERCs which as used as service codes (for example, 911 and 411)

N9X
The 80 of this format's codes are called expansion codes. They are reserved for the time when the current NANP 10-digit numbers must be expanded

37X and 96X
These two blocks of 10 codes are reserved by the Industry Numbering Committee (INC) for unanticipated purposes where it is necessary to have a full range of 10 continuous codes to use.

The rest of the codes - about 680 of them - are available for assignment for telephone numbers to residences and businesses. Among those, there are about 300 being used within the United States right now. Another 40 of those are in use by other NANP countries. Every area code has 7,920,000 telephone numbers available within it.