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make a copy of the credit report that doesn't have them on it. The next time they call, confirm your name, the mailing address and the last 4 digits of your social (you do not need to give anything further). Then inform them that per federal law they MUST send you a dunning letter within 5 days. Do not discuss anything further, simply state you will wait for the letter.

You will send them a debt validation letter, if they don't send you a letter by the 5th day, you include in your letter that they have already violated the law once. If they do send it, you still send the DV. Also in the letter inform them that since this debt is disputed, they may not insert a negative mark into your credit file until they have properly validated.

Then you wait and see what they do.

Basically you want to see documentation that supports their claims. A statement that shows the final balance due, Something that proves they are the legal entity to collect upon the account, a breakdown of how they came up with the amount they say you owe if it differs from the amount of the statement and the signed agreement/contract that allows them to charge such.

Now the statutes are unclear on just what exactly constitutes validation and CAs will often point that out, so I always suggest that in the letter you inform them that you will not pay upon an account that remains unvalidated and in a court of law they would be required to provide such paperwork, no matter if they sued you or you sued them.