Bill payment

I normally pay R299.00 on my cellphone contract but last month R619.00 was debited on my account, and when I spoke to a consultant she explained that I have out of bundle settings , so I was charged extra and I requested that she must change that setting for me as I was not aware of the settings,she told me the system couldn’t allow her to reduce I must call on the 1st of September to change. I managed to download the MTN app and changed the settings on my own. My invoice for this month again has an extra charge and when I called MTN the consultant told me the invoice was already out when I reduced the limit. I don’t understand how can I pay over R250 for a call duration of 7 minutes and worse my bill is the almost the same as the one for August and September I already reduced the settings. I’m not happy with MTN service I really need clarity on this .