When I receive a call from someone claiming to be a collection agency, I write down the information they are claiming I owe. Then I ask them to send it to me in the mail. This simple act alone eliminates a lot of scammers. They do not want to risk sending something via US Postal service because that can trace back to them and add Mail Fraud to their charges.
To add something that was already said, the cc company does not have to inform you, but whoever bought it can also resell it. We have one debt that we have been disputing the amount for almost 5 years. The cell phone company wrote it off and there is now a 3 company trying to collect. I have sent certified letters to the cell phone company and to the first collector explaining that we are willing to pay what we owe, but that amounts to less than 50% of what they claim we owe.