No response to MTN mistakes

When I became a customer of MTN, I took out two contracts, one for myself and one for my mother when she was residing in Orkney. The phones were ordered from MTN Direct. One Samsung Galaxy SIII Mini for 209 per month for myself and for my mother a Nokia Lumia for 159 per month. The Nokia Lumia was delivered to my mother in Orkney while the Samsung was delivered to me in Johannesburg. It has come to my attention that when the phones were sent the sim cards had been swopped around, I have only upgraded my phone and am planning on upgrading my mom's phone at a later stage. However i am now being billed 209.00 for my mother's Nokia Lumia instead of 159.00 as per my contract agreement. I have sent mails to both Lerato and Sibusiso and neither have responded to my request to rectify the matter. The only thing that needs to be done is mom's phone needs to be charged at 159.00 and I need a credit of R100.00 on my account for March. I upgraded the beginning of January, so at the end of January and February i have been billed R50 extra, bringing it to a total of R100.00 extra. I am not willing to pay R50 a month extra for MTN's mistake. HIGHLY DISSATISFIED!!!!