Oh, got it!
I had to enable notifications in Settings/Manage Apps/Phone/Notifications/Phone
Even though the notifications were enabled in general, they were not enabled for the Phone category.
I updated my Mi8 SE from 8.11.15 to 9.6.13 and later to 10.3.1. Both newer versions have a horrible problem. I cannot answer any calls! There is no incoming call UI AT ALL.
See this video:
It is the same if the device is initially locked. I unlock and there is no on-screen hint that someone...
I'm running Mi8 SE with version 8.11.15. When I try to update the phone restarts into Fastboot but after a few seconds the bunny disappears and nothing else happens. The phone simply switches itself off.
Same thing on my Mi8 SE. When I tap 'Download original' for a cloud image in gallery nothing happens.
However, when selecting an image for home screen background they were downloaded successfully.
Factory reset is definitely not a solution...