Bad Service MTN

The service i have received lately is purely pathetic i called in for the second time last month to arrange that my debit order be changed to 25th and yet MTN has decided to deduct when it suits them. What is the point of me calling in to arrange a date when it has not been updated for months???? If i pay in before hand it shows me a discounted price and when i pay that amount i gef notification about arrears but the minute mtn decides to deduct 1 to 2 weeks later they charge me penalties im not happy about this service and will take my business elsewhere when the contract is completed