What do I do?

I have been getting calls from FBCS. I just talked to a supervisor named Kevin. Who was very rude and told me that he could do anything with my debt, he could lower it , he was the one in charge and not to mistake his kindness for weakness, Is this Co. for real? Supposedly I owe a debt to this company, I paid this company off in 2007. So now, because I didn't call and get a receipt back then, that was my mistake and now its a debt still owed. He said he couldn't search the system to see what I had paid. So now I am making a complaint to the B.B.B. What else would you recommend that I do. I've read you comment and now am very concerned about this company. Also is Midland Credit Management the same company as FBCS Inc.?