How to Enable WiFi Calling on your iPhone
1. To enable Wi-Fi calling, go to Settings -> Phone -> Wi-Fi Calling. You may need to enter or confirm your address for emergency services.
2. If Wi-Fi Calling is available on your device, you’ll see Wi-Fi Calling on This Phone. Tap on the slider to enable this feature.
Also, make sure to read the fine print, as they may differ from carrier to carrier. Once you tap the slider, you receive a pop-up message and must tap Enable.
NOTE: The option hasn’t been pushed to all devices for now. You can also call your carrier and check on availability dates as well.