BIGGEST ****MERS EVER

I have a contract with vodacom, am receiving airtime and that airtime is forever disappearing and am always told that its taken by subscriptions while am not using that Sim card to make calls, am using my other Sim as it is a dual Sim phone. I called Vodacom two weeks ago and they said they blocked the subscriptions bt all of my airtime has vanished again and they are refusing to refund it, how sure should I be that they borrow me this airtime and take it again?