Cellc- debit early without authority and still not bothered about rectifying!!!!!!!

On the 21st of december 2017 cell c debited me when my debit order is only set for the last day of every month. They debited me 10 days early without my consent and neither did they send me any correspondence stating that they will debit early. This was confirmed by managers thembe and nomzamo. I have called well over 5 times requesting for a letter to provide to my bank as the debit order went unpaid due to the fact that i only get paid on the 25th. The people at cellc really have a terrible attitude and from the 21st of dec until date i have no letter and a messed up bank account!!!!!!!