Airtime Disapearing!

On Tuesday the 1st of September, I loaded R60 airtime to make important work calls, in less than 5min my airtime finished before I did anything. Then after I switched my mobile data off thinking that my applications were updating in the background, and I bought another R60 airtime and made a call immediately after loading it, and my balance after the call was just above R50 and after while it also disappeared. I called the call centre a number of time and spoke to consultants and they tell me it shows as data usage from their side, but that's impossible if my mobile data is off! I went to the MTN branch in Liberty Braamfontein and they were no help at all. They told me to call 809, I called them and they tell me the same story of data usage which makes no sense when my mobile data is off. Since the 1st of September to date, I have to buy airtime and make a call immediately before it disappears and I've spent more than R300 worth of airtime doing the same thing, since it's the only way I can make calls. Please help me out guys, my phone is useless at this point since I can't do anything but receive calls.