Air Time being used before Data

I've been an MTN contract for quite a while no complaints, for the last week ive realised my air time being used up extremely quick but my data not at all I topped up on Tuesday data and air time to see if the problem would sort it self out. It hasn't I still have data left but my account balance is now R0.51c after purchasing more air time this morning I have tried to contact the call centre at least 6 times today the call gets dropped current data balance 571.89 mb so something is wrong with the system I expect a answer soon