You can tell if you are currently using Wi-Fi calling by these icons on your smartphone display:
Android: The phone's notification bar displays the WLAN connection icon when the connection has been established with the WLAN and the service is operational.
This icon indicates that Wi-Fi calling is operational. Calls can be made via Wi-Fi calling.
This icon indicates that Wi-Fi calling is not operational. Make sure your phone is connected to the WLAN and that the Wi-Fi connection strength is sufficient.
iOS: The notification bar will show "yallo Wi-Fi calling" if Wi-Fi calling can be used.