I have a Galaxy S9 and recently started using WiFi calling. The icon indicates that it is on.
But I'm having minutes charged to my account while I'm in my house and on WiFi (signal is good).
One thing I've noticed is that occasionally I'll pick up the phone while the call is in progress (I do a lot of telecoms for work) and when I start hitting buttons (eg, take off mute) I'll see the message "continue call using WiFi" which implies it had stopped using WiFi and was using my regular minutes. How can I ensure that it only uses WiFi?