fhamator
From time to time I experience this with my devices (P8/P9P on carriers 1&1/Deutsche Telekom) and it seems IMS service registration was lost in these cases.
To re-register my eSIM for IMS service, I enter SIM-APN settings > go to submenu 'APN type' with already opted 'default', 'mms', 'supl' and (un-)check 'ims', save and exit > flight mode on for a few seconds, off again and see if call function is recovered.
If it still ends calls immediately I repeat above steps (undo) and this usually re-registers IMS service to be able to place and recieve calls again.