Afni saying I owe ATT

I just received a phone call from Afni saying that they were trying to collect an outstanding debt from when I had ATT U-verse. Thye told me that I owe $142 for a final bill that I never received from ATT nor did I receive phone calls or leters or anything from ATT. Afni also told me that they were the third collection agency to take on collecting this debt even though this is the first I've ever heard about it. The lady I talked to said she'd settle for an even $90 but I told her I didn't have that money and that I didn't think I owed anything anyway. She told me she put a note on my account saying I was disputing it so I wouldn't get any more phone calls. This was from 3 years ago and I've lived in 3 different houses since then. How do I go about handling this situation since I firmly believe I don't owe anything. Please help.