VODAOM DEBIT ORDERS

Why does Vodacom think they can debit their customers bank accounts when it suits them? I instructed them to debit my account for 28th of every month but they decide to debit on the 1st if the month. Customer care confirmed debit was for 28th but when you speak to the accounts department they say their system shows 1st. No wonder Vodacom is being boycotted!!. I will cancel ALL contracts held with VODACOM when they do become due. I have checked back on all my bank statements as to when prevuous debit orders were taken off which was 28th but now conveniently system changed back to 1st. Vodacom dont blame the machine it is the operator!!