In your search for a reverse phone directory, you will find some pseudo directories promising to allow free phone searches, but they're usually just websites that allow you to search the carrier or issuing location of a phone number, and not the owner. If you want to know the phone number of a person you will use a phone directory.
Enter the number into the box on this page and you're there. After all, there are plenty of free sites for land line numbers. Among these features is the Google Directory, which offers free reverse phone number searches for residential and business landlines.
Cell numbers are generally more private, since you are charged for every call you receive, including telemarketers you don't want to talk to, although it's illegal for them to call your cell without permission. Therefore, if you have an unlisted or private number then you need to find a reverse cell phone directory service that caters to unlisted numbers only. There can be times when cell numbers are included with reverse white pages, but that is rare.
Paid services can provide you many advantages. You get detailed information, which makes it worth the cost.