Wifi Calling Drops or Overridden by Network

This situation occurred on my previous iPhone8 and continues on my current iPhone12. This situation does NOT occur on my spouse's Android Samsung S10.

Home has poor cellular network signal due to radiant barrier in roof (e.g., 1 bar in front of house and "no service" elsewhere in house), thus wifi calling is essential.

Phone is connected to home wifi provided by Xfinity who allows wifi calling. Phone is configured with wifi calling "on" and preferred while roaming. When properly operating, phone displays "XM Wi-Fi".

Frequently (multiple times per day) wifi calling reverts to cellular displaying either "Xfinity Mobile" or "No Service" while wifi and internet are active based on use with other home devices (e.g., spouse's phone, Mac, iPads, Windows laptop). I still have wifi for data, just not wifi calling.

I can usually reinstate wifi calling by either turning phone off/on or by resetting network settings per Apple Support guidance. These do not work 100% of the time.

The problem is that during these "outages" I am missing calls and am delayed making calls until I can successfully reinstate wifi calling.

Previous conversations with Apple suggest it is Xfinity. Conversations with Xfinity suggest it is Apple.

It is my understanding that wifi calling should ALWAYS override the cellular network, regardless of signal strength, with the connected wifi network supports wifi calling (please advise if otherwise). In my case it seems that any momentary cellular signal may be overriding wifi calling.

When the phone goes dark (to sleep) does the wifi shut down such that wifi calling is suspended until woken up by either an inbound call or me waking it up?

As always I appreciate any and all guidance.