I've gathered that various cell networks won't let me use VoLTE on my unlocked phone with forks. I've been reluctant to install LineageOS because of that.
It's the implementation that OEMs used to support VoLTE isn't compatible with AOSP APIs.
If it wasn't for Google here you'd never have VoLTE on custom ROMs, if it exists in any shape or form it's thanks to them.