Mal administration of Customer service

I have been retrenched since Feb 2018. I submitted my documents to MTN and they decline to cover me. They advised that my account was in arrears which was their fault. I have had to call them countless times they were not debiting my account as per our agreement. Now they are chasing me for the balance of about R16000 which I don't have. I have never reversed any of their debit orders, instead they didn't send it through. Same problem I experienced when the contract was due for an upgrade and they fixed it. After the upgrade I had to chase them as they gave another customer my number while I had it. I am unemployed currently and have limited means to be able to pay any account. They need to accept my retrenchment letter and close the account