Time limit is reason you can't phone? (8)
I believe the answer is:
deadline
'time limit' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for deadline that I've seen before include "Latest time for doing something" , "Closing date" , "Time by which journalist must file copy" , "a writer may have to meet" , "Time limit as for journalists filing copy" .)