https://support.apple.com/en-us/HT203032Place a Wi-Fi call from another device (T-Mobile, Sprint, and AT&T)
If you're in the United States and use T-Mobile, you can place a Wi-Fi call from your iPad or iPod touch with iOS 9 or later.
If you're in the United States and use Sprint, you can place a Wi-Fi call from your iPad or iPod touch with iOS 9.1 or later.
If you're in the United States and use AT&T, you can place a Wi-Fi call from your iPad or iPod touch with iOS 9.2.
You can also make Wi-Fi calls with Apple Watch with watchOS 2, or Mac with OS X El Capitan. When you place the call, your iPhone could be right next to you or far away — it could even be off.