@atarilinux
I believe it would be better to use the new port based on android 12 / halium 12 instead of the one based on halium 9.
According to source code, VoLTE is already enabled on this new port.
Therefore, you just need to install Android 12 on your phone, build this new port on your desktop/laptop (or gitlab-ci), install it on your phone, and test if it works.
https://gitlab.com/ubports/porting/community-ports/android12/google-pixel-3a