MTN - debit order over deduction

In good faith, I have a debit order set up with MTN, and last month they deducted an additional R799.00 from my account. Nobody at MTN can tell me what this is for, or find an invoice for this amount. Whether this is incompetence or dishonesty on their part, I dont know? They have an email address called "customer care" which to me should be "We dont care about our customer"