Changed debit order date without prior notice and failing to change it back

In June I changed banks. I then proceeded to update my banking details with MTN. Ever since I changed my banking details they, without prior notice changed my monthly deduction date to the 12th of the month. This is against the contract I signed agreeing to deductions on the last day of the month which is more in line with my salary deposit dates. I have had numerous back and forth emails with MTN and numerous query/ ticket numbers logged and for the 2nd month in a row they confirmed they have changed my deduction back to the contractually agreed date and my deductions are still not going off at month end.

What if I don’t have sufficient funds in my bank account for the monthly payment to be deducted in the middle of the month? MTN will be very quick to suspend service of that happens but they refuse to sort out their mistake.

This is unacceptable service from such a big provider. My contract is coming up for renewal now in October and I’m seriously reconsidering switching to another service provider because of the service they clearly don’t know how to provide.