Wifi calling is not working
You must have a network provider who provides the VoLTE/VoWifi service on your mobile plan.
The VoLTE/VoWifi settings will be in your Sim & Network settings, if its not showing and you have a Network provider that supports it as well as a good 4G LTE signal then try the following to see if this will enable it.
This is for all OP phones, however it discuses the 8 Series, but it should apply to any series as the protocols are the same.
[GUIDE] Enable VoLTE & VoWiFi on all devices NO ROOT.
Log Kit 1.0
There are newer versions but I would suggest trying this with the details provided on XDA.
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by Daniel D., 2021-12-01