BAD CUSTOMERS SERVICE!!!

I have changed my vehicle finance debit order date from the 27 to the 1st of each month on the 17th of May 2018 with told that the amount that is extra amount (due amount) that is due was worked into my installment as this was fine. I received a letter stating what i have to pay the current amount i am paying now without knowing of being informed by standard bank that there will be an additional amount that needs to be paid. On the 17th of June till now, i started getting up to 5 calls a day from standard bank demanding money for an arrears on my account. NOBODY wanted to tell me what it was for and 4 different calls (as recorded) that the money is for 4 different reasons but nobody still haven't assist why i needed to pay this money if they have worked in to my monthly amount i am paying. i have calculated the amount that i am paying extra and it is more than what i am "owning". EVEN WHEN I WENT IN TO THE 6 SIMMONDS , JOHANNESBURG BRANCH TO SORT OUT THIS PROBLEM I FOUND OUT THAT STANDARD BANK CHANGES MY DEBIT ORDER DATE BACK TO THE 27TH OF EACH MONTH WITHOUT ASKING FOR AUTHORIZATION AFTER I HAVE EXPLAINED THAT I AM ONLY GETTING PAID ON THE LAST DAY OF EACH MONTH!!! I have went to the branch in Johannesburg wasting my time as they have assisted me but now i am still getting calls that i owe this money but still the person who helped me told me that he is sending an email (CC me in the email) to the complaints department and that i need to be waiting for their call. 3 WEEKS LATER AND STILL NOTHING BUT STILL GETTING CALLS DEMANDING MONEY AND THREATENING ME THAT THIS WILL BE AFFECTING MY CREDIT SCORE IF I DON'T PAY!!! I have all emails and recording from changing the debit order date till now where i tried calling them and no respond!!! How can you work like this. THIS IS UNPROFESSIONAL AND BAD CUSTOMER SERVICE!!!