Unacceptable client charges

I have had an uncapped 3gig internet line with MTN for over 2 years. I recently noticed that the amount deducted from my account was not what I agreed upon.<br> <br> I received a call from a lady advising me that a promotion ran which would make my subs a lot less. I asked for the documents and before I even got them did she process an upgrade on the account. This for some reason caused my account to reach a credit limit. The account has reached an amount of almost R3500 due by me and to date does the lame excuse not make sense.<br> <br> I requested for this to be corrected and the upgrade was reversed but I am still being held liable for the amount reflecting as due. This to me is just plain unfair and I request that this be rectified.<br> <br> Needless to say I do not wish to further my contract with MTN.