Being charged so much more

I have a huge problem with MTN billing me more than the agreed amount on my contract. I took out a data contract with mtn for R299 per month but they keep on changing me extra. I paid R1100 in July and I just received a bill for R630 for August. I don't even finish the data that they credit to my number so I don't understand why I am being charged so much. I called MTN numerous times and the times that the agents don't drop the phone in my ear no one can explain why I am being charged so much and what these charges are for. I am in desperate need of your help. These are hard times and I need every cent I have. MTN is *****ing my hard earned money and it has got to stop.