How many possible phone numbers can there be in a single area code?

The number of "lines" or "phone numbers" assigned to one area code will vary as a function of the number of exchanges in that area code. Let's look.

In the 555 area code, in the 111 exchange, there are 10,000 numbers that can be dialed. 555.111.0000, 555.111.0001,... 555.111.9999. With 10,000 numbers per exchange, we can calculate the number of numbers or "lines" by multiplication. Look up the number of exchanges in a given area code and make a quick calculation.

For an "absolute" answer, consider that if we could use all the numbers for exchanges (000, 001, 002,... 999), there are 1,000 possible exchanges. With 1,000 possible exchanges and 10,000 possible numbers per exchange, that works out to 1,000 times 10,000 or 10,000,000 (ten million) possible numbers or "lines" per area code.

However, that number assumes that every single possible telephone number is assigned. In the US and Canada, the exchange code cannot begin with 0 or 1, and the second and third digits cannot both be 1 (e.g., 411, 911, etc.). Most area codes also protect prefixes that use the same digits as another area code nearby. For example, area code 415 is adjacent to area code 650, so there is no 415-650 prefix, nor a 650-415 prefix.

That reduces the possible numbers per area code to a bit under 8 million.

In addition, only a fraction of the numbers within each prefix are assigned and in use at any given time. When a telephone line is disconnected, the number is left unavailable for at least a few months to reduce the level of misdials.