There are almost 8 million possible seven-digit phone numbers per area code. Each area code has 792 possible prefixes or NXX codes (for example, NXX-XXXX or 555-1234), explains the Public Utility Commission of Texas. And each "NXX" has 10,000 possible phone numbers attached to it. So, with a little math, we know that theoretically, there are 7,920,000 possible seven-digit phone numbers in each area code. Obviously, not all of these numbers are put into use, so you don't have to try nearly 8 million numbers if you want to randomly dial a friend who lives nearby.