Incorrect billing

Morning. I have been with MTN for years now. best service always, until recently. I was due for my contract upgrade in Nov 2020. Called in October. I have 3 lines all connected to my main line, I pay the exact same amount on time every single month for the last 2 years of my contract of R750. In November I upgraded to a new contract and wanted to cancel my other 2 lines, they said I cant cancel until Jan 2021. OK fine I left it. my new contract is R550 ish. to my knowledge as soon as a contract falls away you pay month to month, which is fine. The one line was a free line connect to my main line and is now out of contract so now I pay R95. which is also fine. I tried cancelling my 2 lines only to be told I am now R1800 in arrears?? HOW If I pay every single month the same amount. and up until Oct account was balanced, then I stop getting statements.... call center said line went up in Oct to R800 plus, then Dec it was R1200 plus... Now its up again. How is my account climbing? I understand the additional few hundred. I will pay this, but somewhere there is a huge amount added. And then also, why was I paid a pro rata for the upgrade? It is not a new line, I understand an upgrade fee, but there should not be a prorata. I have logged and escalated now 4 times. One lady told me they did the refund, but then cancelled the refund again? Can someone from Billing please call me direct that knows what is going on so that I can get proper answers and not jump from one person th the next each time. I need to cancel the data line and change the package for the other line. Much appreciated