Horrible customer service

I am livid with the service from MTN since I have changed my debit order from a cheque account to a savings account. Please note the debit order date has never changed and has remained the same which is the 20th monthly on a savings account . I think about two times I have had to go to an MTN shop to make a manual payment becauss for some reason you fail to debit on the 20th where I make provision for the debit order then mid month on the 12th you go and debit my account and there is no funds. This is bull**** and now my account has been charged interest as if I dont pay but its mtn that is not debiting on the right date. Everytime I call MTN to confirm the date I am told its the 20th, i have even had to accept debicheck mandates twice but still there is no debit order that runs on the 20th. This is not my fault and i am not paying any extra money because of your inefficiency please fix this nonsense. I have attached an sms i have got which says i owe you extra money cause you do not ****** debit on the right date. If this is not sorted within 24 hrs i am taking this further. This is unacceptable.