Placing VoWiFi calls fails without cellular connectivity
Making Voice over WiFi (VoWiFi) calls when there is wifi but no cellular service is not possible. Attempting calls just immediately hangs up in the Phone Dialer app with the message connect to a wireless network to make a call. Receiving WiFi calls does work correctly.
To confirm you are configured correctly with your carrier to allow VoWifi do the following:
Dial *#*#4636#*#* > Phone Information > 3 dots > IMS Service Status > Voice over WiFi and confirm it shows Available.
To test a VoWiFi only call, in Network settings, go to Settings > Network & internet > SIMs > (your) SIM > Wi-Fi calling > Calling preference and set to Wi-Fi only.
If there is still cellular connection, making a call will show the wifi symbol at the top of the dialer. But when enabling Airplane Mode (it isn't easy for me to leave cellular coverage land to test otherwise :-), the Dialer (or other dialer apps) show the message to connect to a wireless network to make a call and immediately close. I did try with the same results with 2 other Phone Dialer apps (including Google Phone app).
I believe this to be a LineageOS bug introduced after LOS 20, but there isn't much (any) traction on their issue trackers: