If you’re traveling abroad and are tired of using Skype, WhatsApp, or Viber to call friends and family back in the US, you now have another option. AT&T launched Wi-Fi Calling, allowing users to call US numbers from abroad over Wi-Fi instead of a cellular network. Provided you’re connected to Wi-Fi, the calls will be free of charge. The service only works for iPhones 6 or newer running with iOS 9.3.

The service also works in the US when you have a weak phone signal

Once you activate the feature in Settings, your phone will use AT&T’s Wi-Fi Calling automatically when you’re abroad, even if a mobile network is available, so you won’t be paying roaming fees. In the US, the feature can be used when you have a weak signal, potentially allowing you to receive calls over Wi-Fi when you’re in a basement or elevator.

Mobile carriers’ Wi-Fi calling services have the advantage of being built in, so they activate automatically and you don’t have to download an app. AT&T’s service is restricted in some countries, including China, India, Israel, and Turkey. And if you call international numbers while abroad you’ll be charged the same as if the call originated from the US. Wi-Fi Calling only works when you call US numbers.