Quirks - IMS
To read about IMS in general, see here. You have been redirected here because LineageOS builds for your device do not support IMS functions.
Why is that?
While it certainly is a standard, there is no standard implementation for it in the Android sourcecode. Some vendors have their own implementation, which is not open-source and can therefore not be integrated into LineageOS’ code. Devices that rely on any of these implementations therefore lack the all features provided by IMS (VoLTE, VoWi-Fi, VoNR, etc.). This might or might not affect you, depending on your provider and region.