MTN doesn't deliver on promises!

I wanted to upgrade my MTN contract to a spesific deal advertised in MTN's January booklet. This was such a struggle because I was on the old system (what ever that means) Eventually after lots of issues I upgraded through the retentions department, and they gave me the deal I wanted by taking a smaller contract and adding on 1000 minutes and 6gb data to the contract at a set monthly price (I have all of this on email) I did not get the correct agreed upon minutes and data and I have called, sent several emails, complained on facebook and basically tried everything to get them to give me the correct minutes and data. No one responds to my mails. The mtn store and customer service line is of no help because I should speak to the retentions department, but they are ignoring all my attempts to speak to them. I really don't know what to do anymore. They simply don't care, but I still have to pay the agreed upon price every month.