dealing with debt validation by afni

About 2 weeks I received a letter from afni saying I owed for a past phone bill. I haven't had a phone in over 12 years. I called them and said to send a validation for a bill they claim is from 1999. So, a few days a got a letter from afni that included the "original statement" (hardly) from the original creditor.

The "statement" says the bill date is from May 08, 2002 (I was not even in my state in 2002) and can prove it. It also says that current charges is a $20.94 CREDIT. Total phone services $20.94 credit. Then also included in the page is a "Total Amount Due $52.34" from somewhere. I called the phone company and they said I did not have an overdue account with them, that I can open a phone account without a deposit, etc.

I had to spend over a year fighting Midland and the crap they pull but this is an odd one. Should I respond to their letter and "validation"? Over the phone they said it was from 1999, but now their letter says its from 2002.