OK, yet another test. I just tried connecting the Quintus to my main UT phone (the Pixel 3a, Focal).
during discovery, the Pixel appears in the list of neighbouring devices on the Quintus (and the Quintus on the Pixel)
when I tap the Pixel in the list on the Quintus, I'm asked to confirm the PIN on both devices, which I did
the Pixel then moves to the "Paired devices" section on the Quintus, shows connected for a few seconds then returns to "disconnected"
on the Pixel, it remains in the list of neighbouring devices (and a notification is re-issued for the last received SMS, which was left unread)
Tapping the Pixel then tapping "connect" causes the following:
the Pixel shows as connected for a few seconds on the Quintus, then it returns to its "disconnected" state
nothing changes on the Pixel on the Bluetooth settings page, but a notification is re-issued for the last received SMS (strange, I know, but I got that same behaviour twice).