No Response on calls logged to MTN

I was contacted by MTN to do an upgrade on my phone as it was due. They went through the process and I received my new device. When I received my monthly invoice however I discovered that they upgraded the wrong nr (not the one they called me for) resulting in an R8000+ migration fee being charged to me. to make things worse, I have been trying to get this sorted out for more than a month now!! Various calls logged to the Customer care center with no response. it is an absolute mission to get hold of an agent. This I managed to eventually do and another week and a half later....STILL NO RESPONSE from MTN!!!!! HOW DO YOU ESCALATE AN ISSUE IF THEY JUST IGNORE EACH AND EVERY MAIL AND CALL!!! I even received an SMS stating that my original call was resolved, nobody even spoke to me.