Free Calls don't work without balance

Hi,

I have some packages of free calls.
If the customer balance is 0, the free calls don't work.
If the customer balance is 0.10, and the cost for minute is 0.10, the call will drop after one minute.

The correct is free calls work with balance 0, if the minutes are consumed and customer don't have balance, drop the call, if the minutes are consumed and customer have balance, debit from balance the rest o call.