MTN PREJUDICED CLIENT DUE TO ******** DEBIT ORDER DISPUTE

MTN loaded an unauthorized debit order on my account after my monthly installment already went off via debit order. Needles to say, this new debit order was returned, since it was ********. I spoke to MTN and they cannot tell me what this new debit order was for. The consultant confirmed that I did not owe them any amount on my account at the time they instituted the debit order. They also say the debit order was loaded through self service, which I did not do. Regardless I should not be penalizes for their ******** debit order. Yet now I am told I must pay R 11.50 for the returned debit order, even if I do not owe them anything on my account. My account has also been changed to a cash account and now I must go pay cash every month. Why must we as clients of MTN be prejudiced? I enquirer from them what this debit order was for since my account is up to date and now I am prejudiced. The response from customer service is also unacceptable.