Stop debiting me on the wrong date

I've had it with Telkom, from their worst customer service to them not knowing what they are doing. I've been in and out of several of their branches in Cape Town but still I have not got the assistance I need. My debit order is scheduled to go off on the last day of the month, but they decide to debit me on either the 24th or 25th of the month. I go into branches, they too cannot explain the reason behind this. I spoke to their billing department but they too are useless. Not only is this frustrating, its destroying my credit score and every time they debit me on the wrong dates I get charge R7.50. Can you guys get it together and sort my issue out.