Debiting on the wrong date

My debit date is set for the 20th of every month. In the month of August I was debited on the 19th. There was obviously no money in my account on the day because I have stipulated the 20th as my debit date. I proceeded to pay via eft on the 21st. I have now received a statement showing that R70 has been added to this months payment due to the “missed payment”. I called the call center explaining that they debited a day before my debit order and asked to speak to a manager. Hope advised that their manager will call me back shortly. It’s been 4 days but it seems my contract is not appreciated. I have been a loyal cell c contract customer for 10 years and I would like to cancel my contract and move to another service provider. This is not the service I expected to receive.