Some services are okay, a lot of them are poor, and only one has left me extremely satisfied. If you have already performed a brief search, you know that there are numerous reverse phone number directories online who claim to be absolutely free.
In most cases you only get information, such as phone provider, location, and so forth unless you use a paid service. Finding it is easy. Imagine your mobile phone number being publicly available, you would be getting a million of calls everyday.
You can use the zip or area code finder feature that some of these sites have. If at all, you are unable to find the name of a person related to a particular number then in all probability, it is an unlisted cell phone number. That's why you'll have to make a small investment to get access to other people's cell phone records.
It is also fully legal and totally confidential. Most of these directories don't give you the same benefits as a paid reverse phone directory.