They said they're "Debt Collecters". Are they really debt collecters or is this just a joke because its annoying. I have like 20 voicemails from them.

Chances are these a phony debt collector scammers.  Better not to talk to them, they will just keep calling and then start with threats. They will often call your family and workplace to embarrass and harass you.  If there is a real debt, you would be legally notified.  Calls, e-mail, faxes and carrier pigeons are not legal notification.