be on call

Từ điển Oxford Advanced Learner 8th

(be) on ˈcall idiom
(of a doctor, police officer, etc.)available for work if necessary, especially in an emergency
•I'll be on call the night of the party.
•The doctor had been on call for 48 hours and was exhausted.
see also ↑on-call _adj.
Main entry: ↑call_idiom