whatupag.com is a reverse number database that allows you to look up numbers and let others know your experiences with that people using that number. You can report phone fraud by letting the community and public know that this is how they tried to get a hold of you. You can find out who's been trying to reach you, even if they don't leave a message, just by looking them up.
Finding out if its telemarketers, surveys, charities, or even debt collectors is as simple as looking the number up in the database and seeing if others have had contact with the person on the other end of the line. Even if you just want to check out a business by using their number, you can see what others in the community have heard or dealt with in the past.