If you have a compatible phone and your phone’s data settings are correctly configured to our mobile data service, then WiFi/4G calling will be automatically provisioned for you, in the background. 

You then just have activate the facility through your phone’s settings: 

• iPhone Users: Go to Settings > Cellular > Wi-Fi Calling and then toggle on Wi-Fi Calling on This Phone.  
• Samsung Users: You’ll generally find WiFi settings under Settings > Networks & Internet > Mobile network > Advanced > Wi-Fi Calling, where you can then toggle on WiFi calling

When you make a call, that call will be routed through either the cellular network or WiFi network, dependent on which is the stronger. 

Calls made over WiFi will only drop if you move out of range and 4G Calling isn't available. If you’re in an area where both Wifi and 4G Calling are available, the call will use 4G first.