I lost the WiFi Calling setting on my Note10+ after the One UI 3 Android 11 update
After playing around a bit I now have the setting back and working -
I went into Settings - Connections - Mobile Networks - Network Operators and turned off Select Automatically.
It then asked if I wanted to scan for networks - let it scan and then tapped on my normal network ( Vodafone ) Then checked in the Phone Settings menu and WiFi Calling option was back !!!
( Turned on flight mode, turned WiFi back on to simulate no phone signal, and call worked fine over WiFi )
Hopefully this might help anyone else having the same problem
NB WiFi calling was already turned on on my network account/ plan
Also while it worked for me and I don't think re scanning for the network would cause any other issues, I take no responsibility for anyone else trying it