Texting and facetiming in other countries with the iPhone

Okay i have at&t, and I have the iphone 4s, its all completely updated and everything. This summer i went to Australia and met some really great people! But right now were only talking by snapchat, which gets annoying after a while. They all have iphones as well, so my question is, does it cost money to imessage them in australia? FaceTime/call? If so, would turning our phones on airplane mode fix that? I read somewhere that you won't get charged if its on airplane mode but that couldve been a myth.
Lastly, do you know of any way if i can't text or call them without getting charged? I could use my landline but thats not very convenient. Maybe an app or something.
please only answer if your 100% sure thank you! Xoxo

You can FaceTime and iMessage for free around the word using AT&T's network or your own or someone else's Wi-Fi (data charges will apply). If you use Wi-Fi, you won't be using AT&T's network.