It is a simple way for you to generate random phone numbers that are based in the United States. All you have to do is selected, which US dialling code you want a set of numbers from or leave blank to create numbers from each state randomly.
The phone numbers are generated at random so it is anyone's guess whether they are active or not. They do follow rules as described in the Numbering Plan Area Code system so they will always be valid no matter what.
The numbers are built as follows: 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 th xxxx Line number or subscriber number [0–9]).