Do you have a screenshot (without your personals) of the bill? It's much easier if we can see what's happening. What was the destiantion you called that got charged?
It is possible the your billing period caused this. Say, your billing is on the 15th of every month. You get 30 minutes for one month and on the last day you exceeded that and overage was charged. Then the next call the following day was part of your next month allotment, so free again. I'm speculating but it it possible.