***** in incorrect billing on my account when I am on a debit order

what a waste of time with MTN, had ***** on my account and reported it to them. today got bills that show me I arrears on over 5000.00 and I am on a monthly debit order via contract. waited 30 minutes just to get through to someone in credit depart. then then told putting me through to Thanda mobeni, I think and went to voicemail. your call centre is a waste of time. please explain how I can be in arrears if I am on debit orders and not one debit order has been rejected. The billing is a absolute joke