MTN still undercharging after multiple queries

I ungraded my handset earlier this year (2015) in May with the understanding that it would cost R449.00 per month. MTN has subsequently undercharged me every month since, debiting amounts ranging from R383.99 (in June), to R145.00 (July and August), to R488.99 (September). <br> <br> I have made muliple queries to try and fix this issue via the call centre (***), an in-store query (***) and again today (***). According to the call centre agent I spoke to today, a 'case' has been opened. But hey, given the current track record, I won't be holding my breath. And even if it is resolved, what concerns me is that I will now be liable for the outstanding monies, although MTN is the one at fault and has failed to fix the issue when it was first reported. But now I as a customer will be debited a whopping amount that I have not budgeted for.<br> <br> So here's what I propose, I should not be liable for the excess that MTN has failed to charge me because I as THE CUSTOMER have gone out of my way MULTIPLE times to fix this issue with no joy.<br> <br> Cell C's contract buyout offer is starting to look mighty attractive.