The only reason you should run into problems with the free service is that if the number belongs to a cell phone. Because there is no central database of cell phone numbers, free reverse phone directories cannot return information on those numbers. You also won´t be able to get information on landline numbers that are unlisted or non-published. This can easily be fixed by a small service charge.
Well, if your still interested, just in case the one bugging you is using a landline, then you can check the service here
Reverse Cell Phone Lookup and Unlisted Number Search: ReversePhoneDetective.com