Overcharging my account

I took an upgrade offer in November, a 2 for 1 deal. The lady assisting me explained to me that I will be paying R379 monthly for 36 months but will pay R576 in December because of the pro rata. End of December my invoice was R1016 and I went to the lady that was assisting me to query the invoice. She logged a call for me and told me to wait 48hours for a reply. A week went by and I went back to find out what was happening but we did not get any feedback. I have since received the January statement and it requires me to pay more than R1900 which is still not what I agreed on with MTN. We called customer service on 12th of January at 16:14 and the lady assisting on the other line told me there is nothing she can do but she saw that I was being billed for the second device which was not on the agreement and she said I must wait until MTN sort this out. When I told her I have waited more than9 days already and the turnaround time was supposed to be 48hours she told me there's nothing she can do and hung up on me. The reference number I have for this issue are as follows: ***- Billing 360, ***- Remedy and # 199733-Zendesk , all obtained on different occasions.