Even if the debt seems real, debt collectors must prove they have authority to collect the debt. This proof could be a statement showing the balance you owe or a copy of the original credit agreement. Even if you believe the debt could be yours, make sure they prove it. They might say they don’t have to give you more information until you pay, but they do.
Be persistent! If they respond in writing and you don’t think the debt is yours, you should mail them a letter (through certified mail with a return receipt) within 30 days of receiving their letter saying that you do not owe the money. This puts the debt in dispute.