What is going on with your accounting?

I have just seen that my account limit has been exceeded. I want to know how this is even possible. I called 808 and get answers that make no sense to me at all. I had .49gig of data left. How can I be charged R850 for data usage? Your call centre agent has no clue in trying to comprehend the problem. I also don't need her ty lecture me on how to use my account. I have been a customer for over 22 years and really don't need to be told about how your accounts work. Your agents actually need to listen to customers and not put the phone down on customers. That is not acceptable. Why was I charged for data and your imposed credit limit exceeded when I try and top up data?