Missing airtime where does it go?

I have noticed time after time that my airtime balance is decreasing. When contacting Vodacom, their call center agents and supervisors always make it seem as if you are wrong. I have complained numerous times and every time they seem to have an excuse. I suggest everyone that has a Vodacom sim please check your balance daily as they seem to pinch a few rands everyday. Over a month it exceeds R200, without monitoring this you will be losing in excess of R200 every month. Vodacom always claims to be SA's best and fastest network but in actuality they are just ordinary *******.