Dan's Samsung Galaxy S21 Ultra keeps defaulting to cellular preferred, instead of WiFi preferred. But instead, he's missing calls and texts as a result. He uses T-Mobile. The cellular is a bit sketchy in his area, so he'd rather use WiFi Calling. But the phone won't let him. Leo says that if he gets out of range of the WiFi, it'll immediately go to cellular. Dan says it stays that way, though, when he's back home. Try rebooting the phone. Often that fixes the problem.
From the chatroom - it appears that T-Mobile is forcing users to use Cellular instead of WiFi calling if the phone determines it's better (even when it doesn't). There is a workaround by downloading the app TASKER. The app will prevent it. Also, make sure to keep updated on the latest Android. Apparently, it locks in WiFi preferred, rather than let the phone decide.