Wrong Billing

On 27 Jan 2015 i called mtn to query my account,, i have been billed R860 and my contract should be R605 with the discounts, after talking to an consultant i was told that it would be sorted out.... On 23 Feb i recieved my new invoice which states i will be paying zero rands and still have a credit, then on the 25th i am again billed for R860, I called 808, it took me forever to get through as the call are continuously dropped, never the less i was not surprised to hear from the consultant that there has been a mistake with my account,I am told by the consultant that i am being billed for BIS????? i have had an iphone since Nov 2014, then i check and confirm my previous contract from 2012, i also upgraded to an iphone but being charged for BIS????, I have not been contacted by anyone from the accounts dept to explain what went wrong or when i will be reimbursed.... shocking.