Incorrect billing

I upgraded 2 months ago to a 20gb data package for R299 p/m,first debit order went of for R1200,I then phoned and they said that I over used data and that I must put a limit to my account and there were additional charges for other stuff aswell, I just left it then as I was furious and confused of what came out of the consultants mounth that was trying to help me,the consultant even admitted that they have made a mistake where I was charged wrongly for upgrading fees and sim swop fees etc,I just left it as I saw that I was not winning here, so I have set the limit to R299 like they have asked me to do and I have asked the consultant if I will ever get a surprise like this in the near future and she said no everything is sorted now,this month debit order went of and again R1200 were debited of my account,I phoned again and they said it was last months data usage, please help me as I have no idea of what these guys are talking about,we monitored the data usage every day and we haven't even used half of the data they gave us. I am definitely not going to just leave it again like last month, and they have everything on voice recordings so they can go back and listen to every single one of those recordings where the consultant told me that everything is sorted and from now on my bill will be R299 p/m as what it is suppose to be ....