Disgusted at MTN Customer Service

I did a contract upgrade at the MTN store in Sandton. The structure of the upgraded contract was I would trade in my old phone and receive a R400 monthly discount for the duration of the new contract. When the first debit order was deducted the discount was not given to me. I proceeded to contact numerous of people at the store and they promised to resolve the issue. 3 months has now passed and the issue is not resolved fully. I was meant to received a credit of R1200 on my mtn account. To date they have only credited me with R516.13 and still owe me R683.87. MTN's lack of professionalism is disgusting. Their customer service is uphauling. Their communication is shocking. I made numerous calls to the consultant handling my issue and to date she hasn't returned my call. What gives MTN the right to just debit my account with an amount that's not agreed upon. I have displayed patience and understanding while they tried to rectify the problem but 3 months later and it's still not resolved is actually sad. As working class people in this tough economy R400 per month is a substantial amount monthly which could have been used for something else.