How can I solve this problem without using the fundamental counting principle?

The 7-digit phone numbers in a city all have 555 as the first three digits. How many phone numbers are possible?

I was told this is a permutationNOTa combination word problem.

In that case, can I applynPnornPr. If not, why not?

WHY WAS THIS QUESTION MOVED FROM BASIC STATISTICS AND PROBABILITY?