First of all you don't need any credits to call or send messages to your FreeTone friends!
You only need "Credits" to call landlines or cellulars worldwide (except US and Canada) and to send international SMS.
Call rates vary according to the country and the carrier. It's easy to check the rates :
From the app left bar menu:
- Tap "Text&Call Abroad"
- Enter the number you want to reach using the international format ( + country code/ number).
You will get the rate applied for texting and for calling (when Country is supported).