I agree totally about not duplicating so will try to answer more fully here than on the other thread I started that requests a carrier list.
I've spent a lot of time on this subject without getting clarity, in fact the first 2 links above are threads that I started. I have contacted 4 MVNOs about this issue, and each without exception, tells me the same thing, that it's something they are waiting for Google to action. So far, none have given any details of what that action is as I can't get past the First Line Support, except with ASDA, more on that later.
On the assumption that all the MVNOs might be correct as they're all saying the same thing, I initially thought that GrapheneOS might treat VoWiFI differently so I asked here too. From the answers on this thread I'm now understanding that Android & Graphene are more or less very similar if not exactly the same in regards to IMS / VoLTE/VoWifi. This means that there's no point in my trying to see if running Graphene on my P4 will solve the problem.
I am expecting a call from a support manager later this morning and will post what he tells me here.
Thanks again for the feedback so far, it's a lot more than I've had elsewhere.