4 months of MTN incompetence

I took out a new contract with MTN 4 months ago, the first bill arrived, and they were overcharging me, i queried this, was told it was being escalated, i heard nothing, next bill comes, still being overcharged, i queried again, get told that the previous escalation was closed, but that they can escalate again, i ask them to do so, i also emailed in supporting documents, i hear nothing, despite sending a follow up email 2 weeks later, next bill comes in, still being overcharged, i query again, get told that the previous escalation was closed, same BS reason, even though i have provided supporting documentation, matter is escalated again, next bill comes in, STILL being overcharged, query again, same story as before, this time i ask to speak to the escalation department, i get told, sorry, i cant, so i ask that they contact me within the next week, well, guess what, still haven't heard anything 2 weeks later, new bill has come in, still being overcharged, and am clearly getting nowhere trying to sort it out with MTN, who is technically in breach of contract.