National Action Financial Services

I just got a letter in the mail regarding a debt...I didn't know I had. The debt is supposedly 20 years old. I asked the representative to send me a validation of debt. The representative said all he had was the original loan. I asked him to send me a copy of that and he refused. In the course of the conversation he referred to an auto loan taken out in 1989 and later on he referred to an auto loan in 1997. Then the representative hangs up on me twice. It all sounds very fishy...he referred to something called gap insurance. I didn't know what he was talking about. How do I find out if this is for real or not. If not, what recourse do I have for stopping them from harassing me?