The first thing anyone should say to a debt collector calling them. The entire conversation should go like this:

"Before we proceed, I would just like to inform you that this conversation is being recorded for my records. If you don't consent to bring recorded, please hang up the phone now, as not hanging up constitutes consent. Can I please have the full name and mailing address of the firm you are calling from? Thank you. Before I speak to you about the debt you are claiming I owe, I need your to send me via certified mail all records indicating that the debt is valid, that it belongs to me, and that you are legally authorized to collect this debt. I will be following up with a certified letter reiterating these points, and will decide how to proceed once I receive the information from you. Please do not contact me by phone again. Thank you."

Follow up with a form letter by certified mail demanding confirmation of the debt. If you don't get confirmation within 30 days off your letter getting there, you can go ahead and dispute the debt on your credit report and chances are it'll go away. If they do respond, and it is actually a valid debt that is in fact yours, decide how to proceed from there.