Dissatisfied with vodacom

Today i loaded airtime and received extra airtime valid for one day which will end at 23:59. So i made a few calls and all the recharged airtime finished leaving zero recharge balance. Some of the extra airtime also finished leaving a balance however upon trying to make calls i get the service which states need to recharge as insufficient funds to make a call yet there is a balance on the extra airtime. Nowhere in the sms does it state any conditions of how to use this extra airtime so it should be that the extra airtime automatically gets used first upon making calls otherwise why give extra airtime???