Unnecessary Overcharging on my Contract

This is sophie Telkom account NO:***1. It is so painful to pay what you shouldn't be paying. This is the 9th month I requested Telkom to debit their installment on the 20th of every month but they attempt debiting on the 15th of every month and those days I don't have enough money in my account. Every time they try debiting and couldn't find any funds they charge me a penalty. My monthly instalment is 907.00 but I end up paying 2077.00 monthly because of the penalties they are adding up while it is their fault for debiting on the wrong date. I'm not sure if they are doing it intentionally or what. this is killing me really because even on their system I have updated the 20th. whoever I find when I call Telkom, they even attest that the debit day saved on their system is the 20th not the 15th. Paying this amount monthly is killing me because it is a 2 monthly installment but because of unnecessary daily 105.00 charges from Telkom, this 2077.00 is for 1 month. sometimes I blame myself why did I moved from Vodacom to this Network...I cant wait to finish my contract and move back.