AT&T wants money after 3 years?

I got a collection letter from "NCO" Collection Agency for $57.00. I called them and they said it from AT&T "final bill" from 3 years ago. I requested a copy of this "final bill" and was told that they are no longer on file and could not produce one "BUT" I still had to pay the money or my credit would be compromised! They told me that AT&T could not find me to give me a bill. I have not moved in 30 years, had the same phone number for 30 years but switched carriers after the "OLD AT&T" became the "NEW AT&T" and raised prices over 50%. These phone companies just beat you down with charges that are not legit! You can't fight them as no one will answer your calls or questions.