Ridiculous billing

MTN billing department is a mess, I recently upgraded my contract with them. My previous contract was a R349 a month, upgrade on the 13th of November to a R249 package, my surprise the invoice comes and the total bill is R906, I called 135 no assistance they tell me of a pro rata, I think they think I do not know what is a pro rata. Went back to wonderpark to query about this, the lady who helped me to upgrade did not even want to talk to me. Now I am stuck with a ridiculous bill. The best I can do is to reverse the amount, if I do not get any response by the end of this week, I am reversing it and I am ready for the outcomes. I can see MTN is ripping people in the name of pro rata.