All I can say is, if you don't want to be hit with a lot of charges, KEEP the phone in PLANE MODE.
Even if you don't answer or make calls, AT&T will charge you to KNOW that there is a VM, and it is basically a round turn, meaning two call charges, usually at 1.19$ a minute or more.
When I go international, I DO use my iphone, but I turn it to Airplane mode when I get ON the plan
Then, when I land, I will turn ON WIFI, but not the phone side, so KEEP it in airplane mode. You can then connect to WIFI hotspots (they are really everywhere) and get email, use applications, download stuff, etc. you can even use SKYPE and other apps to make calls back home for free or for like .02$ cpm. this is really great. People can even REACH you as long as you have a skype out number and leave the app on.
I also use the skype product to CALL MY VOICEMAIL to hear them and delete them, and then you can be fairly safe to turn on the phone side as long as you are comfortable with the pricing in the country you are visiting.
But, I pretty much just use the WIFI side for email, texting, tweeting, apps, internet, etc.