How to Enable Wi-Fi Calling on iPhone
Step 1. Open the Settings app and go to “Phone”.
Step 2. Tap on “Wi-Fi Calling” and toggle the switch for “Wi-Fi Calling on This iPhone” to enable the feature.
Step 3. Read the confirmation dialog and tap on ‘Enable” to enable wi-fi calling, you’ll then be brought to a few pages of terms and conditions and important details* about the wi-fi calling feature dependent on your cellular provider, agree to the terms to continue enabling the feature.
Once enabled the Wi-Fi calling button will be green.