I've been struggling with this problem for months on both of our iPhones. Apple, please listen....I will also post a link to this in a ticket to them too. My phones both drop WiFI calls frequently. I'm in a pickle because due to a radiant barrier in my house, I get 0 bars of cell service, so WiFi calling is the only thing I can use and I've used it for several years (4+) without too many issues until about 4 months ago. Many times the line seems to go dead and then the call drops, other times the call just drops, and other times I hear the person on the other side but they can no longer hear me....and then the call drops!
iPhone Xr #1 - iOS 14.4.2, WiFi-Assist OFF (although it happens with it ON too!)
iPhone Xr #2 - iOS 14.4.2, WiFi-Assist OFF (although it happens with it ON too!)
Spectrum Mobile (Verizon underneath)
Spectrum Internet
Apple Time Capsule Router, not in bridge mode
Based on a post here I just turned off 5GHz on my router which will "steer" everything on the network to 2.4 GHz including my iDevices that I use for wifi calling. So if handling "switching between 5GHz and 2.4GHz" is the issue then this should be a temporary work around. We'll see. I'm not too hopeful.
BTW, I contacted Spectrum Mobile who sent me Apple Support who sent me to Best Buy who sent me to Spectrum Mobile. The nice thing is that the second Spectrum Mobile associate sent me the link to this thread.