Allied Interstate/Palisades/AT&T

Late last year (Nov 06) I received a letter from Allied stating I owed money to AT&T & that they were willing to settle for 1/2 the amount. I called the number to find out what this was about. I had a cell phone in 1996-97 w/ ATT. Upon speaking with them, I told them that I was not aware I still owed money to them & was a bit confused. Honestly, I have NO idea if this debt had been paid. I don't see why I wouldn't have as i paid off ALL the other debts from that terrible time in my life. Anyway, I received several more calls from them in the past few weeks & I finally decided to answer. I told the man that I do not think that I still owe this debt & that they need to provide me w/ proof. I told them I called ATT & was told there was not an outstanding balance (which is TRUE). He stated that Palisades paid ATT & they acquired the collection from Palisades. When I again repeated to him that he needed to send me proof in writing that I owed the debt, he hung up on me!!! This was almost 10 years ago!!! Again, I'm not sure if I owe it or not, I think I did pay it off. Regardless, isnt the SOL up? I think it's 5 years in FL. What should be my next step? A validation letter? Thanks for all your help! Feel free to email me!