There is a setting in the WiFi Calling that allows you to set this as the preferred method for calls when connected to WiFi. This works for me without any issue.
Thanks for the reply I had already set preferred method to WiFi but still keeps switching off, only way to get it back on is to reboot phone but then switches off again after a few hours.
So I've just discovered if I switch Nord VPN off I can switch WiFi calling on again just by clicking the icon in the quick panel, I couldn't do that before.