VODACOM AND MTN - *****ing money from clients when airtime is purchased.

My housekeeper uses Vodacom, she has complained that she loads eg; R30.00 airtime and before even making a call, the airtime has gone. She actually changed service providers from MTN to Vodacom because the same thing was happening. Her son is on MTN and the same thing is happening. I would like a response from both service providers as I and they feel both providers are *****ing from them.