Got replies from Fairphone support (indeed - they do reply):
Fairphone 3 is technically capable of using VoLTE and VoWiFi technologies but we are still waiting to complete the partnerships with the various phone carriers. At this moment we do not have an exact indication of when those partnerships will be finalized.
So @AnotherElk was right in a way: Fairphone and Hollandsnieuwe apparently have to arrange a partnership to make VoLTE work. And VoWiFi, but that’s somehow already working on my FP3 