MTN is doing as they please

I have a contract with mtn and they debit my account every month on the 26th for their payments. This month they suddely sent me a statement with an amount I did not understand, i higher amount almost double my installment. I called them, and the lady I spoke to said she logged a query for escalations and she’ll sent me an sms with the ref number (of which i never got) and said I will get feedback within 2-72 hours (never got it). Then days later I called again, then the lady I spoke to assured me they are going to debit R397. But this morning I wake up to a debit order from MTN of R779 which I never even authorised, which I don’t know came about. I mean with all the financial crisis we as individuals are going through, they just do as they please.