MTN won't tell me why my debit order amount keeps changing.

The service at MTN is in declining. My contract was for data at R83. When it expired they told me that contracts are now 3 years long and the price will be R119 per month. They billed me R150 that monthe because I was out of contract. I asked them to make it a pay as you go contract but they offered me R49 from a different department I think. I paid that for a few months but this month they billed me R109. When I query the change they tell that I am out of contract. I've been out of contract for 4 months so I don't need to be told that. It would be nice if the answer my question as to why they have charged me 3 different amounts is 4 months. The last call I made the lady put the phone down on me.