Incorrect Billing

It has been 3 months in a row that I have been incorrectly billed more than 100% of the amount I am supposed to pay. I am still billed as though I am paying for a device yet I did not renew my contract. The walk-in stores cannot help and all they do is call call centre and you are made to speak to an agent. I have been told it takes 72hours for queries to be solved yet 3months later all I keep getting are reference number. I seemed to know more about the prices of the products they offer more than the agents helping me. The amount this far being charged can pay for about 12months of my bill. This is frustrating as there is no one who can actually do their job or help. I have been told I will only be charged R99p/m excluding CLI but yet I see myself paying hundreds of rands. Is there an agent who can help rectify this mess.