Over the years, I've often wondered why my mobile phone doesn't do this "WiFi calling" thing. Being at a conference and not being able to make a call from the underground parts of the Barbican Centre (didn't think of that in the 1970s, did they) finally prompted me to get it fixed.
A big shout out to this chap, who has figured out the right incantations to enable this with a few well-hidden settings and a reboot. I can confirm that in my case, the desired icon appears:
And it works! Where previously I got bleating about "mobile network not available", I was able to make a five minute call which sounded clear as a bell. Calls don't stay up when moving between mobile and WiFi (even with 'WiFi preferred' turned on), but it's still a big leap forward over not being able to call at all - these days, there are lots of locations like remote holiday cottages where WiFi is no problem, but no mobile signal is to be found.
I'm not sure which of OnePlus or Three to blame for this not Just Working by default, but at least it is possible to kick it into life in a few minutes.
Update, 2019-06-22 - I ended up turning off "WiFi preferred" as it meant that moving from my desk to the kitchen at work to take calls without disturbing people would cause the audio to break up - probably because that journey puts me at the other end of the building to the WiFi access point which serves my desk. Hopefully increased adoption of Unifi kit at work will allow seamless roaming in future.