On iPhone SE (3rd Gen) with iOS 15.7, Verizon WiFi calling keeps switching between WiFi and cellular which is very week in our house. It goes back and forth even during the same call and the cellular portion is very poor connection with dropping audio. WiFi signal is very strong. Why doesn't it stay on WiFi. This didn't seem to happen a while ago.
I know how important it is to get the best call quality, Sandywhite. Your phone is designed to work using the best possible connection so it would switch between cellular and Wi-Fi calling when signal is stronger. Have you tried disabling cellular data so it stays on Wi-Fi during a call? I suggest doing that when you are connected via Wi-Fi and test it out. Settings-Cellular-Cellular Daya-off