t310 - all callers suddenly displayed as 'unknown'

Since yesterday, all incoming calls I get is registered as 'unknown',
instead of displaying a name from my phonebook or number.
(I'm not sure if 'unknown' is the exact term used - I'm using a Danish
translation.)

I've tried browsing the menus to see if there's something I might have
accidentally tampered with, but I can't find anything.

Has anyone got a solution for this?

Best regards
Mads