NPA Numbering Plan Area Code: Allowed ranges: [2–9] for the first digit, and [0-9] for the second and third digits. When the second and third digits of an area code are the same, that code is called an easily recognizable code (ERC). ERCs designate special services; e.g., 888 for toll-free service. The NANP is not assigning area codes with 9 as the second digit.
NXX Central Office (exchange) code: Allowed ranges: [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).
xxxx Subscriber Number [0–9] for each of the four digits. This unique four-digit number is the subscriber number or station code.
Sounce: Wikipedia
NPA Numbering Plan Area Code: Allowed ranges: [2–9] for the first digit, and [0-9] for the second and third digits. When the second and third digits of an area code are the same, that code is called an easily recognizable code (ERC). ERCs designate special services; e.g., 888 for toll-free service. The NANP is not assigning area codes with 9 as the second digit.
NXX Central Office (exchange) code: Allowed ranges: [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).
xxxx Subscriber Number [0–9] for each of the four digits. This unique four-digit number is the subscriber number or station code.
Sounce: Wikipedia