@gandalf I can say that the first three items on your list do not occur for me on a Oneplus N10. I just tested to make sure. There are some times when it does seem to maybe get stuck reverting to 3G for calls (I now see an H in top indicator, for HSPA I guess) but WiFi doesn't trigger it for me.
@gpatel-fr I have also experienced the odd ring when dialing out since my OP N10 got VoLTE, but mostly not.
I thought maybe it was only occurring when I was connected over 3G, but no, after dialing some random 1-800 numbers it happens the same for a given number whether I'm on VoLTE or 3G/H.
I don't really see lack of ring as much of an issue (I'm still just amazed and grateful to have VoLTE!) but it's still worth trying to figure out when it does/doesn't happen. I'll start looking for whether its network dependent.