still awaiting 500MB and no response

I received a call from Nirvana from MTN Yello mobile ensuring me that I would receive the data within 24 to 48 hours of subscription. Ive received sms' saying order being processed and how change APN but nothing stating activated. I changed the APN still nothing. I have been calling the call centre and sending emails to no avail. I have only received one requesting my details and that was it. I do not expect to now be charged for something I am yet to receive.the most annoying part I that there is no response from the call centre and only one number yet when they want your business they will call even when the call gets cut you get a call back. I ended up buying 1 gig of data and now dont need the 500mb until this has finished so I dont want to be billed however cannot get hold of them to find out the status. They have my account details which is dangerous! I need some assistance.