There are other reports that Deutche Telekom’s Volte and L5 aren’t compatible, see: Incoming calls in Germany - O2 -> no with vodafone -> YES - #2 by jonsger and VoLTE support progress - #447 by st.boom
This seems to happen around the world. Just need to test others (and other users can give hints here what works or not).
[Edit to add: I don’t think this has ever been explored further as the workaround (changing provider) has been easier, but I wonder if it’s connected to what was shown in IPV6 only hotspot and IPv6…? If you want to start troubleshooting from somewhere. Or it may be that operators just aren’t allowing specific types of modems to their networks.]