Bad Service

I logged a request on 20/04/2015 to convert my contract to prepaid as of 01/06/10. No response to date. Thereafter I contacted the call centre on 2 occasions and was finally advised that it will be handed over to retetions and was assured that it will convert to prepaid. I have now received an invoice for the month of June. The past 2 days I have tried the call centre 4 times, and each time I held on for over 20min and the call gets dropped.<br> How am I going to get to resolve this without being able to speak to someone at Mtn.