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Use "on call" in a sentence
"—Sir, Dr. Carrothers was the doctor on call tonight, but as I told you, he had a sudden emergency."
"It just so happened that elite neurosurgeon E. Malcolm Field was on call that night."
"I love that her best friend is the wry and acerbic and twice-her-age Iona, who is always on call to dispense maternal affection and wry, acerbic advice, and goes with her to cool clubs to drink sodas through straws and hear cutting-edge bands."