You can calculate an Erlang equivalent for VoLTE quite easily.
For Huawei it is L.E-RAB.SessionTime.QCI1/3600 .
Session time above is measured in seconds, hence dividing by 3600 (secs in an hour). So if you held a VoLTE call for one hour, session time would be 3600s and hence our formula would give you 1 erlang (one “circuit” held for one hour).