what happening to my vodacom prepay phone balances

I want vodacom to explain what is happening to my vodacom prepay phone balances. I made recharge to call my wife yesterday I used up 20 minutes by using unlimited vodacom voices 10 minutes twice. That was the only call I made late in the night, today as I made attempt to buy power bundles I got message that I dont have sufficient amount to buy even the R2 voice for 10 minutes. I checked my airtime balance and found I had only R1.55 I want vodacom to account how ca I buy airtime make 2 calls that I dont exceed the 10 minutes period for each calls and later the next day all my airtime balance is gone. what is going on with this airtime. Its pretty clear someone is preying on the airtime balances. I am very clear on this as I have made this same observation previously here on hallopeter and I never got any satisfactory response or closure on this matter from vodacom. Can vodacom please come out clean to say what is happening with airtime is there some other method that vodacom is using to eat into ones airtime and force us to keep re-charging endlessly