Unmissable date — and phone not working? (8)
I believe the answer is:
deadline
'unmissable date' is the definition.
I can't explain the rest of the clue.
(Other definitions for deadline that I've seen before include "Time-limit as for reporter's copy" , "Latest time for doing something" , "Latest time for completion" , "Time-limit as for journalist's copy" , "Target time" .)