On iPhone:
To turn on Wi-Fi calling follow these steps:
Step 1. Go to Settings
Step 2. Select Phone
Step 3. Click on Wi-Fi Calling and enable WiFi Calling
You might need to enter or confirm your address for emergency services.
If Wi-Fi Calling is available, you’ll see "Wi-Fi" in the status bar while viewing Control Center.
Then your calls will use Wi-Fi Calling. When cellular service is available, your iPhone uses it for emergency calls.
If you turned on Wi-Fi Calling and cellular service isn't available, emergency calls might use Wi-Fi calling. Your device's location may be used to aid response efforts when you place an emergency call, regardless of whether you enable Location Services.
For more information about iPhone setup, visit https://www.apple.com
On Samsung:
Step 1. Open the Phone app, and then tap on the More options (the three vertical dots) icon.
Step 2. Select the Settings menu.
Step 3. Find and tap the switch to turn the Wi-Fi Calling feature on.
Alternately, you can also enable Wi-Fi Calling from the Quick settings panel. Swipe down from the top of the screen, and then tap the Wi-Fi Calling icon to turn it on.
When cellular service is available, your iPhone uses it for emergency calls.
If you turned on Wi-Fi Calling and cellular service isn't available, emergency calls might use Wi-Fi calling.
Your device's location may be used to aid response efforts when you place an emergency call, regardless of whether you enable Location Services.
For more information about iPhone setup, visit https://www.apple.com