Credit Management, LP Just received a collection letter for a bill ,I paid, to Qwest 7 years ago Carrollton Texas

I just received a collections letter from Credit Management, LP stating they are collecting an unpaid debt I owe to CenturyLink (Formerly QWEST).  I had Qwest service in Idaho from 1992 to 2007.  I moved from the area and had all of my mail forwarded to my new address.  I paid all of my bills and have never in 7 years gotten any from Qwest or Century Link saying they showed an amount due on my account.

I called Credit Management to try to get this cleared up, and they were not interested in anything I had to say.  They just kept repeating, "This is a valid debt, and you owe this money."  I asked them if there was any record of Qwest or Centurly Link attempting to contact me in the past 7 years, and they just kept insisting I owed the money.

They told me I had to pay the money unless I could send them proof that I paid this bill 7 years ago.  In the past 7 years, I have changed banks and accounts.  It will take a lot of time to come up with the documentation they require to get them off my back and keep my good credit clean.

Is anyone else having this experience with them?  Is there a class action lawsuit in process?