First of all, calling FreeTone friends is free worldwide and doesn't require any credit.
Receiving calls on your FreeTone number is always free, and doesn't require credit.
You can also call landlines and cell phones, your friend doesn't have to be on FreeTone. Depending on destinations, calling is free or requires credits.
For more details on how to earn credits, please check the "How to get credits?" topic.
Please check "Where can I text and call ? " topic for a list of Countries.