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This just prolongs things, but what you should do is send them a verification letter - send them a letter by certified mail, return receipt requested, that you want the debt verified, a copy of the contract and copies of the statements.
Until they send you the information they can't contact you again. However, that does not mean that they can't file suit on the matter if they so desire.
This does give you some breathing room and it will get you copies of the contract and statements so that you can review such.