From the Phone app, tap the Menu icon.
Tap Settings.
Scroll to Wi-Fi Calling.
Tap Wi-Fi Calling.
Tap or slide the Wi-Fi Calling switch to On.
You may be prompted to add an emergency address when enabling Wi-Fi Calling for the first time. Emergency services use this address to locate you if you place an emergency call over Wi-Fi. You can add or update this address at any time by tapping Emergency address from this screen while Wi-Fi Calling is enabled.
You've completed the steps!
