[Jeffry]

Save all the used numbers in a file. Then, when someone buys a new number, loop through the file's context and use strcmp between the line of the file and the number (formatted to strings [wiki => strcmp helps]).
If the file is read to the end, and the strcmp was never same, then he/she may use the number, in case that there is a line same to the number, stop the loop and return that the number is already in usage.
Got that? ^^

Jeffry