Incorrect billing

In 2016 August/September I app**** for a tablet contract in MTN Vryheid. There was a massive delay in the delay of this delivery, until December 2016/ January 2017. The delivery company in Richards Bay called me to arrange a delivery and I told them that I had relocated to Mpumalanga Nelspruit in January 2017 and that I no longer needed the phone. In March 2017 I received my MTN bill with and additional +/- R600, I have been paying this additional amount since March 2017 to date. In June 2017 I enquired telephonically about the R600 and I was told that I can ONLY enquire at the Vryheid branch. I called the branch and they promised to call me back with an explanation. In August 2017 I made another telephonic follow up in the Vryheid branch and I am still waiting for the response to date. In the meantime I am paying +/-R600 for a Tablet that I never received.