¡No So Fast!
Well, your problem will be already solved if you are looking for a land line, but you could be interested in finding a prank caller, doing his call from a Cell phone and in that case, this "reverse directories" will not be useful. furthermore, if the number you are trying to investigate is "unlisted" you will also be unsuccessful with this type of search.
The Solution to discover who is the person behind any phone numbers lies behind a service called: Reverse Phone Detective, which is probably, one of the best services around to keep databases from various sources to make fast and accurate searches. Now, buy this kind of databases and integrate them into one single and searchable form is a gigantic work, so this is the reason this people charge $19 dollars for the service.
There are similar services in the market right now that offer the same type of service, but this is one of the most accurate around. Although you will not always be sucessful using this service, it will give you the most chances of sucess and most users are very happy with the service.