I Had the same problem, and I am a controlled testing freak! 🙂
I had the same problem when I was in my home, or on wifi. I have the latest update on my 6 plus and my watch! and T-Mobile is my carrier. The resets are a waste of time, in my opinion. I did the following four tests and repeated them three times with same results each time;
1) Wifi on, wifi calling on, Bluetooth off = call doesn't work, call doesn't work, call doesn't work
2) Wifi on, wifi calling off, Bluetooth off = call works, call works, call works
3) Wifi on, wifi calling off, Bluetooth on = call works, call works, call works
4) Wifi on, wifi calling on, Bluetooth on = call doesn't work, call doesn't work, call doesn't work
Call works = Alerts and answers properly on Apple Watch
Call doesn't work = No alert at all on my Apple Watch
My conclusion, is my watch always receives the call when wifi calling is turned off. Now it should work, in my opinion, when wifi calling is on, so I have submitted this to Apple.
IF you are using wifi calling, try turning it off to see if that helps. You can still stay on wifi, turning off wifi calling doesn't affect that.