Changed debit order date

Received a sms from Absa bank to inform me that n debit order from Mtn went off on my acount and had to be reversed due to insufficient fund. After calling Absa I was told that it was Mtn who changed my debit order date from the first of every month to the 29th without my concent or knowledge! Me as customer is embarresd and angry at this and not hapy that its gona be my responsibility to reverse everything and pay the extra costs as I only get payed on the last day of the month. NOT happy!!!!