My educated guess is that it is used without a hyphen when used as a verb, and with a hyphen when used as an adjective.
Dr. Smith is on call. He is the on-call physician.
Similar to follow up:
Dr. Smith will follow up with the patient when she has her follow-up visit.
It is not hyphenated.
No it's not hyphenated.
Motorcycle is not hyphenated
words are only hyphenated when they have a separate meaning when separated than they do when hyphenated
No, "hardworking" is not hyphenated.
Yes it should be hyphenated.
It can be. Some words can be seen both hyphenated and non-hyphenated.
No. Well above is not hyphenated.
The word desperate hyphenated
peace of mind is not hyphenated.
"Field" is not hyphenated.
Yes it should be hyphenated.