Customer service

Mtn has been over billing by three times more than I normally pay for the last two months, I have been trying to fix the issue for the past 7 weeks. Nobody in store can help me and I have been to a few branches, and they all tell me to call costumer care, with was in active when mtn was on strike witch is unacceptable!!! Now when I call my call gets dropped!! Or I'm on hold for 20 minutes or more, and then my call gets dropped! I Im outraged . Mtn is one of the worst networks I have delt with , and what to cancel my contract , but can get hold of them to do that either! Has been a stressful situation that at the moment seem impossible to resolve! Which is now affecting my life, because of my monthly budge. Very unhappy costumer!