NCO and Americredit

I have been receiving calls from NCO but when I answered the phone it was just a recording asking us to call them back. They just called again and I answered and I got to talk to a live person. She said it had to do with Americredit. We voluntarily had our car repo'd last year. I asked the lady if they could validate the debt. She said what do you mean? I said I want debt validation. She said we already mailed that to you. I said well we havent received anything. That is no lie either because I havent gotten anything in the mail from them. She said it was mailed out on the 20th of this month and that I should be getting it. Ok so my question is what am I suppose to do when I get this validation? Not really sure what I am looking for here. I hear that NCO are hard to deal with and I dont have any money to pay this debt. I also dont want to get sued! Any help with this matter will be greatly appreciated! I live in Illinois!