From my understanding it is mostly a Samsung thing. If you search XDA by phone model and look for the “Official” LineageOS releases then look under “What Works” and “What Doesn’t Work” it should tell you per specific model.
It is a tough nut to crack (otherwise LineageOS devs would have done it) but if you look on /e/'s hiring page they are specifically hiring VoLTE engineers so hopefully the 2021 timetable fits.
Here is an example regarding XDA searches from the previously menioned thread👇
