If you think that you paid the debt, or simply don’t owe the debt claimed, you should write and request the collector provide you with validation of the debt, and a detailed itemization of any amount they claim owed.

If you do not think you owe the debt or disagree with the collector’s calculation, you should try to provide documentation of payment showing that the debt was paid, or why you don’t owe the amount claimed. This letter should be sent to the collector by Certified Mail, Return Receipt. 

Be sure to keep a copy of all correspondence with the collector. It is important to keep a thorough paper trail of your dispute.