No feedback on over-charging on monthly bill

I phoned the call centre as they are billing more than the contract amount that I upgraded to in May. Previously they said that due to me upgrading early, I am still billed for the old handset for another 3 months during the new contract. This was news to me as no one ever explained this. It is now November and I am still paying for the old handset, 6 months later. I phoned again and they transferred me to the "Solutions Department" reference number 1/ ********** ********** . After 25 minutes on the line, they explained that some one will phone me within 7 days. This never happened. They admitted that I am paying too much but cannot correct it. Why is this so difficult to correct? I will now cancel my contract when my term is over and move back to MTN. Very bad service.