Billed almost Double for 3 months with no refund

I was due for upgrade on one of my account and noticed that from the month of June until August was billed to almost double for no good reason. My understanding was that with most network provider if you haven't decided on what upgrade you are going for you must be charged what was currently being billed for previous months. The only explanation `i received was that the rates went up but which doesn't make sense especially on that specific account `i had, i ended up cancelling my contract and going to cancel all my account i have with MTN, as i was advised that there will not be any refund from them as they charged me the correct amount according to MTN, which is not entirely true.