I called them only once to inquire what they were trying to collect on, and when he gave the details of the account and the original debtor I said I had never had a card with Cap1 and that they would be hearing from me. The rep I spoke with was actually nice, he wasn't rude at all, but tried to tell me it was my responsibility to go to the police and file a fraud claim. Since it is their responsibility to prove I owe the debt, I sent them a Debt Validation letter (DV), asking them to send me various information, such as a signed contract, account history, how they calculated the amount owed, their license to collect in my state and proof the debt was within statue of limitations to collect, etc.
I requested they only contact me in writing, cited regulations in the letter and asked them to respond within 30 days per the law.
You can find examples of DV letters in these forums. Unfortunately, I do not have them linked, still not that swift using this forum