Very bad service from MTN

My friend Mr V Tonder recently did a migration on his account as it was time to upgrade and he did not want to upgrade but wanted a cheaper package. During the Migration MTN removed him from debit order and put him on cash payment. He called them and requested to be put back on debit order. He has not missed a payment as the debit orders always went through and the two payments he needed to make via cash he did. How ever MTN keeps saying that He owes them more than R 1000,00 that is in arrears. After numerous calls no one at the customer service can tell him why the system says he is behind because according to them he is up to date with all payments. The 135 centre can't say whats wrong and they put him through to legal and legal does not answer the call.