HUGE INCORRECT BILL

After 4 years of using MTN as my 3G service provider (on a standard 2G contract per month), I have received an unusually high bill in October 2015 (over R10 000), well in excess of my usual R120 a month. I contacted the call centre and received a mail from their data validation department showing the websites from which these alleged software downloads were made. Mty IT department has however produced evidence proving that this is factually incorrect as the downloads were not executed from my PC at any point. I have since passed on my IT department's response to MTN and after calling the call centre for over 3 weeks have not received any feedback to resolve the matter only to find that my account has now been debited with this huge bill, which is grossly unfair given the matter hasn't been resolved. In all my calls this past week to the call centre I have requested that senior management be consulted to resolve however to no avail- promises are made but no-one ever calls back. I am rather frustrated and need to know how to take this further- would greatly appreciate an urgent response!