NO proof that I owed

I got a notice to pay a phone bill from 1993 with NO proof that I owed it. I've read a few of the other complaints and some sound very similar to mine. The question is - what good does it do to complain here? What happens if you don't pay? How can they ask for 14 year old money when the FDIC only makes banks keep info for 7 years? I have not found a way to prove I paid it or not. Any suggestions?