Not being Billed Correctly

I've done an Upgrade on 18/02/15 to a MTN Sky and I've take an Iphone 6 Plus and the Monthly cost will be R2099. I received my first bill and the amount was R3968.50 did not thing to much of this amount as I was under impression that it could be for a late upgrade fee my second invoice was for an amount of R5117.36, I then knew something is not right, I've contacted the Call centre on 01/05/15 and I was informed the correct department is nor open on a Saturday but they will Log my Call and that it will send to the correct department that will attend to it on Monday. I've then contacted the correct Department on the 12/05/15 but noting was done as yet but is was addressed as urgent, I've contacted the Call centre again and I was informed that it still not resolved but it was transferred to a Manager that's working on it as we spoke. I've contacted the call centre last week but with no luck as they were striking. I've contacted the call centre today and as no surprize it was still not resolved, my reference for these Calls is ***. I request that my contract to be cancelled due to MTN not keeping to the agreement and I've given them more that reasonable time to resolve this issue.