Mtn will not covert expired contract.

My contract expired on the 15/05/2015. I called MTN on the 15/05 and requested that my contract be changed to prepaid. I was then told by the call center agent that their retentions team does not accept calls and that she would send them a request to call me back in 3 - 5 days. This did not happen. She gave me a reference number if i had not received a call from them which is - ***. I called them back but kept being cut off. This is unacceptable. I have been trying to convert my contract to prepaid since the 15/05/2015 to no avail. I have also been going in to every MTN shop that i walk past to see if they can do it but no one can help and they keep pointing me in the direction of the contact center. I tried calling them today and was on hold for 20 mins with no answer. My airtime has now been topped up again and I will now be debited another month because MTN will not assist. I am aware of a debit that it due on the 30/05/2015 but I will not pay another month as this is not my fault because I have been trying to convert my contract to prepaid since the 15/05/2015.