my airtime keeps dissapearing

Everytime I buy airtime I never get to enjoy it coz it just disappear. Tried calling the call centre which is of no use coz they can't tell me where the airtime goes to. I think MTN is *****ing my airtime because how is it possible that you can't tell me where the airtime went but you can tell how much I load and who I called. Can someone have a look at this and call me.