Bad treatment

I called Vodacom to change the debit date to the 15th of every month and that was done for 2 or 3 months but in August Vodacom did not take their money on the 15th and I was not even aware that the money was not debited. On the last day of the month they tried debiting my account but there was no money, why was the debit date changed to the last day of the month in the first place and who changed it because it was not me. Now this is affecting me in a bad way, the bank is charging me for unpaid transaction and it is not my fault that Vodacom decided to debit on the wrong date. My credit score is also affected by this. Today I called Vodacom and no one could confirm why my debit date was changed back to the last day of the month. Am I supposed to pay double on my next debit date for someone else's mistake? Is that fair? I have been a Vodacom customer for too long but I am treated like nothing and as if I am not a loyal customer. Do you even know how must the bank charge for unpaid transactions on the cheque account?