Samsung devices have a problem with VoLTE on custom ROMs as their VoLTE implementation is proprietary. A forum 
Search #samsung-galaxy + #volte will give more detail. For a long time it was thought reverse engineering their methods would be too difficult to incorporate into builds but some builds appeared on the forum this week [UNOFFICIAL BUILD] Samsung Galaxy S9 (starlte) / S9+ (star2lte) / Note 9 (crownlte) for /e/OS-a16 3.x.
Samsung phones will work in regions where carriers offer 2 or 3G without VoLTE.