Not a happy client

On the 25th of July 2017 I was charged more than the amount that I'm usually charged. Im used to paying R453 every month since the past 12 months so this month I was debited R872 so I called Mtn call center and the lady who helped me told me that I was charged for discounts which I'm not suppose to be charged for. They further told me that the matter will be resolved . No feedback was provided ever since so I called back on the 27th I was told the matter is under investigation I called again on the 1st of August 2017 and I was told the same thing, what surprises me is that I have been receiving the same bundles since last year but now I'm being told my contract changed after 12 months and I was not even informed. Today I received my statement and I'm still expected to pay the R872 . I called and again this is the 5th consultant who's assisting me . She told me that my query was logged on the 10th of August hence I'll be charged my problem is how can my terms and conditions change without me being informed as a client, and how am I expected to pay for something that I'm not even receiving. I'm not Happy at all as a client what's even worse is that none of their call centre agent can help with my query.