Unresolved billing problems

In December 2014, I upgraded my alarm system with ADT. At the time I was incorrectly billed and the sales rep (Mark Schoeman) ensured me that he would have the billing rectified. I have followed up with Mark via email each month as my statement still reflects a balance outstanding. The last response I received was in March assuring me that the technical manager would authorise the credit. Today I receive an sms that my account will be handed over at the end of this month. I honestly don't know what to do! I have been fighting this for 6months and its not my fault. furthermore, no one responds to my emails and no one picks up the number *** that I was directed to. I am exasperated beyond belief and this is a waste of my time!