CHANGED MY DEBIT ORDER DATE YET MTN STILL DEBITED

I firstly struggled to change my debit order date using 808 call centre therefore I visited Fourways mall branch and was assisted efficiently by a young gentlemen to change my debit order date from the 25th to the 1st of each month on Saturday the 28th of March, I even received a confirmation sms that my debit order date has been changed to 1st from 25th. I was unaware of this until I looked through my bank statement today that MTN still debited on the 25th, I tried calling 808 several times I keep holding until I am in a ghost line, no operator available to assist me. please attend to my query asap.