Airtime just dissapears

My airtime just dissapears, this is probably the worst network ever. I have set my out of data control so I know my airtime does not get used when I don't have data. But still when I have airtime available say like R 36.20 then a few hours later, and my phone was on charge (no one had access fo it) it now has R 12.60 availeble, and I still have data on it, when you call customer care they don't give you an option to speak to a consultant. I will not recommend this network at all.