Epic 100 package unfair

i just got a contract upgrade that they give you r100 airtime to use in a month , the airtime gets finished u can't load prepaid airtime must phone and ask for a limit increase at a additional cost , i loaded r100 more i,m sitting on a wifi connection didn't phone or sms anyone i get a sms that says u got r24 left to use so r75 just vanished into thin air , so every time now i must phone in and ring up a huge account for airtime i can't account for? don't want this contract anymore , how must i work if the airtime finishes and then the outgoing calls just gets cut and i can't top up??? ....