I got to know that it does not have anything to do with the Vodafone network as previously said by people, but it has to do with the capabilities stated by Fairphone and the OS as delivered by them: The Fairphone OS does not ship the VoLTE and VoWiFi capabilities as stated in the Fairphone shop (see also the answer from Fairphone support in the article How to use VoLTE on FP3). Therefore Fairphone must deliver its OS to Vodafone where it is checked for Google certification, enhanced by the named capabilities and tested afterwards.
In last months there seemed to happen a problem in the agreed process between Fairphone and Vodafone - maybe caused by the Corona homeoffice rules on both sides. Now, even without your intervention on Vodafone side, this process is ongoing again and I hope that a new update can be delivered in the next few weeks. Additionally there is an improvement plan to make this process faster and easier, so that updates can be provided in time.