How Turn off iPhone's Cellular Radio to ensure Wifi calling is used? Is this possible?  If so how is this done?  Can this be achieved by clicking on the aircraft icon to put the iPhone in airplane mode?  Assume device is using IOS 12.

Ensuring that calls are made through wifi and not the cellular system is important when roaming.

Assume the it is possible to turn of the cellular radio and forcing wifi callig: Will this prevent the iPad from ringing?
To restrict telephony to wi-fi only, display the Control Center (on my SE, I slide up from the bottom of the screen to display Control Center), select "Airplane Mode," and then select "Wi-FI" to turn wi-fi on.

I don't know if it'll disable an iPad from ringing.