INCORRECT BILLING AFTER UPGRADE

I did an upgrade in October and was told that I would be paying R579 per month for the phone and package I had chosen, I got my first bill after the upgrade and it was R764.75 - R185 more than I was quoted. I phoned MTN and they said that the relevant discounts had not been app**** and that this would be rectified and the extra charges credited. This has not been done. I have checked on my MTN app and it is still showing the inflated amount and then my unbilled costs which will be next months bill is also showing R764.76. I have phoned MTN again and they said I must wait and see what my invoice reflects next month and take it from there. I don't want to do that, I want it resolved NOW.