AT&T says calling someone via Wi-Fi from one US phone number to another US phone number won't incur any additional charges when abroad. However, calling an international number from a US phone number will still incur the usual international calling rates, but it won't cost you airtime charges.
"You can make calls to domestic numbers and receive calls with no additional charge and no impact on voice call airtime usage," according to AT&T's website. "For international calls, the rates from your calling plan or international long distance package apply."
Wi-Fi calling was first introduced to iOS in October and was previously limited to the US, Puerto Rico, and the Virgin Islands. Wi-Fi calling is not only useful for saving money, but it also provides better call quality in areas with weak cellular signal.
AT&T customers with an iPhone 6, iPhone 6s, iPhone 6 Plus, or iPhone 6s Plus can also send text messages via Wi-Fi for free while international as well.
The feature is currently restricted in China, Cuba, India, Iran, Israel, North Korea, Pakistan, Saudi Arabia, Sudan, Syria, Turkey, the United Arab Emirates and Vietnam, but available in many others.
If you have a supporting handset, enable Wi-Fi calling on iPhone by visiting Settings > Phone > Wi-Fi Calling and enable the "Wi-Fi Calling on This iPhone" option. iOS will then automatically route calls and texts over the cellular or Wi-Fi network.