DIFFICULTY CHANGING BANK DETAILS

I have sent MTN numerous emails to change my banking details as my previous bank account has now been closed. I received one email from Pumeza Mabuya a week ago requesting certain details, to which I immediately responded. Since then I have received no further responses to any of my emails ensuring that my bank details have been changed so that my debit order can go off at the end of the month. You would think that something like this would be attended to immediately to ensure that they would receive payment. Why should I, as a paying customer, keep following up to make sure someone else is doing their job!?