Absolute shocking service

I regret the day (25 April 2015) I app**** for a wireless phone with Telkom. I have received nothing but absolute shocking service since I received my first invoice. Firstly I was charged R1100 for the actual device, which according to the plan I am on, I am not supposed to pay for it, only R399.00 per month for 24 months. Next up Brenda calls me and asks when I will be paying this R1100, and says I have to pay it because their enquiries department has a major back log, and then they MIGHT credit it, when they get to my issue one day. Now I got an invoice stating that they actually credited the incorrect charge for the phone, and after payment made I am still owing them R1300? I would like to know how that is possible, because R399 a month for 3 months (May, June, July) is only R1197 and maybe add R150 for the last days of April I was able to use the phone, and that does not even make it a TOTAL of R1400. And after paying R700, it should be NO MORE than R600-700 outstanding. NOT R1300!!! And I have sent Brenda an email on 16/07, 20/07 and 21/07 without any response. Even asked for the correct persons details if she could not help, but she just ignores me . USELESS!!!