To find the identity of a caller by using his phone number, you need a reverse lookup, or reverse white pages. There are several reputable, and free, reverse lookup sites online. You may have to endure ad pitches offering to give you more information if you spend some money, but for land line-based calls, a reverse directory should get you the information you're looking for. If the number you need to match with a name belongs to a cellphone, the process is not quite as clear-cut or reliable.
Visit the reverse white pages directory at www.whitepages.com/reverse_phone. Enter the phone number, including the area code, and click "Find."
Visit anywho.com and click "By phone number" under "Find a person" to begin a reverse lookup search.
Get creative looking up cellphone numbers. Cellphone numbers aren't listed in standard directories, so it's unlikely you'll be able to perform a successful reverse lookup -- although you'll find plenty of sites willing to charge you for the attempt. General Internet searches on a cell phone number might yield the right information, as can searches on social networking sites such as Facebook. As of the time of publication, there is no one, free, reliable resource for reverse cellphone lookup.