These are the specifications for wifi calling with iPhone.
With Wi-Fi Calling, you can place a phone call if you have a Wi-Fi connection in an area with little or no cellular coverage. Learn how to make a voice call using Wi-Fi Calling.
To place a Wi-Fi call, you need an iPhone on one of these carriers:
iPhone 5c and later are capable of Wi-Fi Calling. Check with your carrier on which models are supported.
Place a Wi-Fi call from your iPhone
Turn on Wi-Fi calling in Settings > Phone > Wi-Fi Calling. You might need to enter or confirm your address for emergency services.*
If Wi-Fi Calling is available, you’ll see Wi-Fi after your carrier name in the status bar. 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. Emergency calls might send your device’s location information to help emergency workers find you, regardless of whether you turn on Location Services. Depending on your carrier, emergency workers might use the address registered with your carrier.