Air time Query

I have a top up contract with MTN and I have loaded R600 worth of airtime in the last 3 days and it has just been getting used up without me using my phone. I have contacted the call centre and after been on hold for 40 mins and then 31:45 mins later I finally explain the situation to a rude consultant and the problem has still not been solved. my fone is being used for business purposes and I would really like an explanation has to where the hell is all this airtime going too...