How to see who is calling from that unknown phone number

Getting a phone call from a number you don't recognize can be annoying. Sometimes you simply don't know whether you should pick up or not. Is it the National Weather Service calling to inform you that they chose the name you submitted for the next hurricane, or is it a debt collector that wants to know what you are up to? Or perhaps just a friend who switched to a new number?

There are reverse phone lookup web sites that can be of use in such a case, and WhoIsThisPhone is one of them. The service has been online for only a few months, yet over 1.3 million queries have already been processed.