No one knows what they doing

I signed a contract with MTN for R449.00 p/m and went all good until one month to date my debit order is different every month. I went to MTN Shop Kolonnade to inquire and the lady told me that my usage limit was not set to Zero or was, I dont know. She promised me that is fixed now and that the debit will be normal again. This morning I phoned MTN Call Centre because my debit was different again. Thando Ngwenya told me that she is guessing that the prices went up with R10.00 so I asked if it was so why would my debit be different every month? And my wife is with MTN also and her debit is staying the same. Now all of a sudden it is USSD Charges for my cellphone banking even though I have Minutes, Airtime and Data on my phone I still get charged for USSD. My wife also does her cellphone banking but her debit stays the same. She said that it was linked to my contract. I've been banking for a few years now and this never happened. I asked her to get me someone who can fix this because I never agreed to that and she just left me hanging for 20min and never told me whats shes going to do. I eventually hung up because no one was going to help me. I want answers and I want this fixed.