DEBIT ORDER DATE CHANGE

I contacted cell c on 05/05/2015 to change my debit order date from the 20th to the 25th of every month as my salary date has changed. I was promised everything went through and my new debit order will be the 25th. On the 20th of May my account was debited and I contacted cell c and I was informed that it takes 30 days to change, fair enough I agreed. On the 22 of June the debit order went off again, I again, contacted cell c to find out why is it still going off on the wrong date. I was again informed that on their side (cell c) it has been amended and changed and apologised and promised that the next month it will go off on the 25th.<br> Today the 22nd I received an sms from the bank that my account is overdrawn, to my surprise, my account has been debited again on the wrong date.<br> My concern is my credit with the bank is being messed up every month, I am always on the negative because I have not done the provision for THEIR debit order. Help me to sort out this mess<br> <br> Please note that I have 2 lines with cell c and the other line was changed a month later but went through successfully without any hassle, my problem is on the other number<br> <br> Thank you