I have "enabled" it successfully in a dedicated profile. I can see the option to configure it and also downloaded the required language packs for speech recognition. Everything shows up fine, however, when I do receive a call, it is always routed to the default GrapheneOS phone app and not to the google phone app.
I have set the Google phone app as standard in this profile, but it doesn't change anything. I have then stopped and disabled the GOS default app. When I then receive a call, no app pops up and I only get a notification about a missed call.
So I suspect that the phone call is always routed to the GOS phone app in each profile and you need to set everything up in your main profile. This is however someething I don't want to do, as I want to keep it empty and only use less-privileged user profiles.