With the latest software update for your phone you can use Wi-Fi Calling when traveling outside the U.S. This means you can call back to the U.S. or receive incoming calls, with no airtime charges. When you call international numbers, your rates will be the same as those in your smartphone plan or your international package.
It seems that international support is limited to iOS 9.3, but it remains unclear if that is the case as previous version of iOS supported AT&T Wi-Fi calling. AT&T has also updated its Wi-Fi calling webpage indicating support for international calling. While Wi-Fi calling is available in most countries, AT&T notes that the service will not available in China, Saudi Arabia, India, Turkey, Israel, United Arab Emirates, Vietnam, Pakistan, Cuba, Iran, North Korea, Sudan, and Syria.
Wi-Fi calling launched with iOS 9, letting users make/receive calls and text message when there is no service available. You can learn how to enable Wi-Fi calling using our tutorial here.