Hey there and welcome to the Samsung UK and EU Community Forum
Depending on your device (as you haven't mentioned which device you have) and your location....
Have you selected "WiFi preferred" or "mobile network preferred" in WiFi settings?
To access WiFi Calling settings, pull down from the top of your screen, press on WiFi Calling and select settings.
I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
My device is an S25 Ultra.