Data voice call between two apple phones My girlfriend and I will be travelling in USA and using ATandT pay as you go sim cards while there. The only problem is that we don't get International calling.

So I want my gf to be able to use data to void call family on their iphones.

But I don't know anything about apple, so I am not sure what is the best method. What is important is that it uses data and does not default to the cellular system and cost us mucho denairo!!
Am I missing something obvious here or can you just use FaceTime Audio? Obviously it won't work if you use the keypad, but you can call from anywhere you have their contact card (Contacts, Phone, Messages, FaceTime, even Mail...)

You did say both parties have iDevices, right?