This follows the rules outlined in the North American Numbering Plan at https://en.wikipedia.org/wiki/North_American_Numbering_Plan.

The NANP number format may be summarized in the notation NPA-NXX-xxxx:

The allowed ranges for NPA (area code) are: [2–9] for the first digit, and [0-9] for the second and third digits. The NANP is not assigning area codes with 9 as the second digit.

The allowed ranges for NXX (central office/exchange) are: [2–9] for the first digit, and [0–9] for both the second and third digits (however, in geographic area codes the third digit of the exchange cannot be 1 if the second digit is also 1).

The allowed ranges for xxxx (subscriber number) are [0–9] for each of the four digits.