Refuse to cancel contract / Incorrect billing

My contract came to an end in Jan...In Dec 2015 as per their rules and regulations, I gave them a months notice to terminate my contract as it was the end of the contract in jan 2016. I faxed and emailed their retentions department as this is what I was told. I completed all their forms and submitted it with a copy of my ID. In jan my contract was not ended and moved to prepaid as I had requested and I received a bill now billing me for all items separately which was part of my package as TELKOM MOBILE did not CANCEL contract as requested. I phoned call centre and was told that they would elevate the query to a manager. I phoned the CALL CENTRE 3 TIMES for the same thing as NO_ONE came back to me. I then went in at BRANCH LEVEL at Galleria mall - where the assistant then ALSO Scanned in all documents necessary for RETENTIONS department as there was NOTHING he could do at BRANCH Level...Now I receive a message to say that my service will be cut due to non payment. well - I wanted my contract moved back to PREPAID in JAN already and NO_ONE is complying wth my request!!!@!!