When I had sprint, I had seamless transitions between wifi and mobile. If my network got weak/ slow it would transition to mobile mid call. Idk if it ever went from mobile to wifi, but now that's its t mobile the wifi calling experience is much worse.
In fairness, I haven't tried it on my new s22
I thought these were temporary in that they last several decades, not several years. Permanent school buildings are intended for several generations.