settled debt

Hi!

I had an account with direct tv over 4 years ago.  I cancelled that account and there was a remaining balance.  I was contacted by a debt collector 2 years ago and settled the debt with them for a portion of the amount that was originally due.  I received in writing that it was "settled" and direct tv accepted the money.  Fast forward 2 years later and I am now recieving phone calls from another debt collector who is telling me that Direct TV is sending me back to collections and I must pay the remaining balance.  How is the possible if if I reached an agreement already and it was accepted?