Incorrect billing for 7 months!

I renewed my contract with MTN November 2014 and upgraded to an Apple i6. The phone was faulty and had to be returned. After a month and no replacement (despite paying for a phone I was not using) I agreed to take an alternative phone which resulted in a contract adjustment as the subscription was less. To this day I'm still paying a premium rate as the subscription has not been adjusted after 8 months. I phoned 2 - 3 times a week and have merely been referred from pillar to post. It's now going on 8 months, I still phone on a regular basis (sometimes no one bothers to answer at all) and I'm told due to striking there may be a delay. I'm talking 8 months of unacceptable service!