Account moved to legal department without notice

I was utterly shocked when I found out that my account was moved over to legal. I done a upgrade on 03/12/2015. New contract amount is R799 and old was R329. When I received my invoice in December/January it reflected an amount to of just R329 of which I paid the following month I then received a invoice for R799 of which I paid. Then I noticed that my simcard wasn't work anymore. When I called 808, they said that my account was moved to Legal due to an outstanding amount for December 2015. When I checked my 2015 Invoice I saw that the invoice had changed to R799 owing leaving me with an amount owing to MTN. It is unfair for me to pay R799 for a new contract that I didn't have for a full month, My cycle runs from the 14th to the 13th. I left a message thrice for the Legal department to contact me to explain. No one has gotten back to me. MTN is dragging my name through the mud for a mistake that they have caused on there billing. I demand a explanation and a apology for this and I will use my Consumer Protection rights and report this.