Android includes a setting to make Wi-Fi Calling the preferred option over cellular, but Verizon hides it. If you are willing to do a fairly minor hack of your Android phone, you can change this setting.
Instructions are at: How to Enable the Preferred WiFi Calling Option on the Verizon Pixel & Pixel XL
The article refers to Pixel phones, but it worked for my Galaxy S8. One difference is that I had to use the 'settings list global' command while in 'adb shell' to confirm that the setting had been changed. The 'exit' command will take you out of 'adb shell'.
It would probably be prudent to check that the changed setting survives your first Android update (other than security updates) from Verizon.
Note - We have just enough cell signal at our house that our phones would spend 10-20 seconds trying to connect using cellular (unsuccessfully) before giving up and switching to Wi-Fi Calling. By this time most callers would have hung up since they heard only silence.
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