Debit order never changed

I went in to Tygervalley Vodacom to change my debit order two months ago and they said that it will change for July so July the debit order will go off at the end of the month and not on the 25th. It was never changed and the debit order went off on the 25th but was rejected and I had to pay for that. It cost me R120 for that and I want to know how Vodacom is going to correct this problem and make sure the debit order is changed. This is utterly unacceptable