Mtn Cellphone Contract Query

I had a cellphone contract with mtn, then I was retrenched from work and I did shared the relevant document about my retrenchment with mtn but mtn did blacklist me. Now my account is handed over to Nudebt I tried to make payment to clear my name but after I paid up the balance which was shown to me from start, they then come up with the new balance. Please assist me.