A further note...
Never, ever agree to pay someone during a phone conversation when you have no idea who they are. OK, they have your account info. But, do you know if they are legally allowed to collect the debt? They are required to provide you with proof IN WRITING before you pay them.
Research DVs on this site; there's a template in the do-it-yourself section which you can find a link to at the top of this page.
Write down the company's name, ask for an address. If they will not provide an address, ask for their phone number if you don't already have one. You can then google the company name and/or phone number to get a mailing address. Or post the company name and/or phone number hear, we can help you find an accurate mailing address.
Only communicate with them via writing, and make sure you tell them (again, in writing) that all communication must be done this way, not via phone. That is your legal right.
Every letter you mail them, make sure you sent it certified return receipt, and hold onto the signature card you get back in the mail.