Cell CANCELATION DEPARTMENT still haven't cancelled my contracts since July 2007

I cancelled my mobile contract back in 2007 and was told i'm owing still, i paid in full but never received a paid up letter. Guess what same account was/has been handed over to Normal Bissett & Associates for collection more than 10yrs after settling the account. Maybe there's something i missed please assist since debt collectors cannot all they want is for me to pay.