All is sweetness and light...it does help to 'talk to someone' - even if no-one (immediately) listening!
0.11.0 worked like a dream!

Addendum 2: Of course, it could be BT, but I thought I'd check-in with you guys first before tackling the behemoth; but to be fair once you get to a actual person, I have found they were knowledgeable and helpful. Anyway, as the problem hasn't been reported here, I'll adduce it's some glitch involving BT and close this question; and my life can move on. Thanks for the (tacit) help.

Addendum: Text messages are usually ok, but one took ages to complete sending: indeed I eventually cancelled it. Apparently it was received 17 times.

My new Fairphone 4 with Ubuntu Touch installed, incoming voice calls do not ring, but go straight to BT voicemail, which produces the expected text message. Calling from my landline, the phone rings, as expected. The phone settings are:
• transfer all calls: off
• transfer if I am on another call, if I don't pick up, if phone is unreachable: all on.
When I try to change the parameters on to off, it says this cannot be done at this time (meaning, at any time).

All is sweetness and light...it does help to 'talk to someone' - even if no-one (immediately) listening!
0.11.0 worked like a dream!

I have now found stable v 0.11.0 and am trying that.

I have encountered two problems installing on a new Fairphone 4:
Summary:

1. installer reports error: Cannot read properties of undefined (reading 'action__wait')
2. It seems ubports installer has a bug. I tried following its complaint that this was a beta version and to install the stable verwion, but then snap said it was the stable version and that there was nothing to download. This is a contradiction.

1 with ubports-installer:
Error: unknown: TypeError: Cannot read properties of undefined (reading 'action__wait')
stack trace: TypeError: Cannot read properties of undefined (reading 'action__wait')
at /snap/ubports-installer/435/app/resources/app.asar/src/core/plugins/index.js:81:29

2. with snap:
   ubports-installer complained about its being prerelease 0.9.7-beta and I'm to issue 'snap refresh ubports-installer --stable'; when I did -> 'snap ubports-installer has no available updates'.
   So, I try to install it from snapcraft.io/install/ubports-installer/ubuntu#install - that is, using 'sudo snap install ubports-installer', but -> already installed. So I try to re-install it by 'snap remove ubports-installer' but -> base directory not empty. Issuing 'snap install ubports-installer' -> it's already installed; I issued ubports-installer and -> no such file or directory.
   This is a contradiction.
   I issue 'ls -l ubports-installer' -> 'drwx . . . 418'. I issue 'rmdir 418' -> can't remove, not empty. I issue 'cd 418;ls' -> empty.
   This is a contradiction.

I had used the installer successfully on different makes of phone.
Please help!

@mrt10001 Success! I tried what you suggested but I had no luck. I tried what the twrp site said and again no luck. I flashed Android v9 and started again. And that worked. Thanks so much for your help, MrT; it got me started!

@mrt10001 OK! I got to fastboot mode, then turned phone off: work for tomorrow! as it's 8:30pm in UK now.

@mrt10001 Thanks. I tried that but no good.
The screen displayed says 'Your device is corrupt and can't be trusted and may not work properly'. I pressed for 10 seconds and screen went off but came on again of its own accord. I turned it off and pressed on before it came on, and back to the same screen.
So I can't seem to get to fastboot screen.

This is a compilation from github 'Pixel 3a XL unable to enter the Ubuntu system #2759'

Chanshunli posted:
Unable to enter the Ubuntu system, ubports installer directly entered UBports Recorvery after installation.
There followed a picture of UBports recovery screen, with logo etc in Yellow. [Too big for inclusion.]

I added:
Using installer 0.9.6-beta:
/tmp/last_kmsg has, at end of ile, many 'cannot get SElinux label on /dev/block/mcblk... device no data available'.
But the colour was orange, not yellow; and Slot b
It seems the phone cannot now be switched off. From the screen with the message 'Your device is corrupt. etc' pressing vol down and/or power results in screen going blank for about 1 second, but then the message screen reappears.

I tried the solution to post: 'Stuck on Google logo instead of recovery', but no effect (older version after all).