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.
The best way to contact Oncall interpreters is through the contact section of the Oncall Interpreters website. One can choose to call Oncall or to contact them through a form online.
Yes.
It is not hyphenated.
Motorcycle is not hyphenated
No it's not hyphenated.
words are only hyphenated when they have a separate meaning when separated than they do when hyphenated
Yes it should be hyphenated.
No, "hardworking" is not hyphenated.
It can be. Some words can be seen both hyphenated and non-hyphenated.
No. Well above is not hyphenated.
peace of mind is not hyphenated.
No, "homework" shouldn't be hyphenated.