They are in violation of multiple aspects of the Fair Debt Collection Practices Act with the statements they made to you and it is doubtful they have any ability to sue you in court.  If, or more likely, when they call back keep demanding written proof of the debt and proof they have a legal right to collect that debt in your state sent via US Mail as required by the above law. Do not acknowledge that you owe the debt over the phone. You can also remind them that federal law requires they send you the required proof within 5 days of initial contact.  Keep logs of dates and times of calls and the content of the call, it could come in handy later if you decide to pursue action against them for FDCPA violations.