MTN poor service

I have been a MTN customer for past 12 years now. In June 2018 I was contacted by MTN to ask if I want to upgrade my contract. I upgraded to a new package. As part of the upgrade I asked for a new phone and was quite specific about the phone that I wanted as they have sent me the wrong phone before. The MTN consultant assured me that I don’t have to worry and that she will make sure I get the right phone. Low and behold when the phone was delivered it was not the phone that I asked for at all. I returned the phone unopened to one of the MTN branches in Cape Town a few days after it was delivered. Since that day MTN have billed me monthly for the phone. I have emailed their customer service and the MTN branch numerous times and also revisited the branch to ask them to credit me for the amount deducted for the phone so fat but to keep me on the current contract package. Despite all this no one has contacted me and MTN keeps on billing me for a phone that I returned to them. This is very poor service. I am extremely frustrated with this situation. Please help.