Someone is not doing his or her job between NuDebt and MTN

I currently have a contract with MTN sometime my account was handed over to NuDebt without my knowledge and that was after I settled the amount I owed but continued paying well, all of a sudden am being harassed by messages of NuDebt saying that am owing I have to settle the amount, how now because without fail MTN receive it's money yet am being harassed for people who did not do their job right. My name is blacklisted can't even open new accounts because am being flagged. Once confirmed my settlement with the mtn legal department and indeed was cleared of not owing my account but to my surprise since March been receiving threatening messages from your collections department, there is someone not doing his or her job here😡