AT&T has expanded its Wi-Fi calling feature to allow subscribers to make calls to the United States over Wi-Fi when travelling abroad. When the service was initially launched in late 2015, AT&T subscribers could only make calls over Wi-Fi while they were in the United States. Puerto Rico, and the U.S. Virgin Islands.
So, now when you are travelling abroad and are in an area with poor network reception, you can use Wi-Fi calling to call back your friends and family in the United States for free. If you end up calling a non-U.S. number, you will be charged as per AT&T’s international call charge rates.
AT&T has also updated its website to reflect this change, and it clearly mentions that iPhone 6, 6s, 6 Plus and the iPhone 6s Plus support this feature provided they run on iOS 9.3. AT&T is also pushing a new carrier update to iPhones to enable this feature, so make sure you have it installed as well.
Additionally, using NumberSync AT&T will now also you to receive or make calls over its data network from other devices that are linked to your iPhone and account, even if the latter is switched off or in a different location.MacRumors]