The music quiet when phone call thing is particularly attractive. I think I have most of the pieces of it here. HomeAssistant, Owntone, airplay (shairport-sync) speakers, a phone running the HA client.
I think the "ringing" state provided by the companion app only works on Android, not iOS though.
OwnTone has an HA integration so I would expect you can do something similar with that.
HA Owntone integration works very well as does shairport-sync & its mqtt interface. Owntone also does Spotify.
Thanks for your thoughts btw.