be on call
if someone such as a doctor or engineer is on call, they are ready to go and help whenever they are needed as part of their job

[TahlilGaran] Dictionary of Contemporary English ▲

on call
available and ready to work or do something at any time.
Working for Mitnick was a lot like being a doctor — I was constantly on call.

[TahlilGaran] English Idioms Dictionary ▲

on call
to be available to be called to go to work
His job is to repair computers and he is always on call.

[TahlilGaran] English Idioms Dictionary ▲

on call
adj. phr.
1. Having to be paid on demand.
Jim didn't have the money ready even though he knew the bill was on call.
2. Ready and available.
This is Dr. Kent's day to be on call at the hospital.
The nurse is on call for emergency cases.

[TahlilGaran] English Idioms Dictionary ▲