incorrectly billed 5 months so far - almost R3500 owed to me!

I upgraded to a Made for Business L+ sim only deal for R699, but I am being billed R1377.99 every month. MTN has acknowledged the error, in writing via email, several months ago, but continues to bill me incorrectly. They have also not credited me the overcharge. This has been ongoing since September 2023.