Best way is imho to ask the man who made the port for your device, if VoLTE is yet implemented into the port and how normal behavior should be during a voice call.
VoLTE is a protocol and has to be implemented into software/OS, so 4G capable hardware is always hardware-compatible with VoLTE. Yes, as far as I know, in 5.0.0.67 it should be implemented.
What does the display say during voice calls? Does it change to 2G or 3G during calls and after the call back to 4G? Do you have some VoLTE settings options in Settings menu? Is your device port 64 bit or 32 bit software?
There were situations reported earlier, where some settings not matching the network providers parameters and so connection couldn’t be established.
What does this mean? You have to set VoLTE to ON if you want voice and hi-speed data in parallel.
I hope, someone here will answer, who understands more than I. Good luck!