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ready to serve when called. I live a very hard life. I'm on call twenty hours a day. I'm sorry, but I can't go out tonight. I'm on call at the hospital.
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.
Available if summoned, as in Medical residents are required to be on call at least three nights a week. This expression originated as at call in the late 1500s. Also see at someone's beck and call.
1. Available when summoned for service or use: physicians who were on call for 48 hours.
2. Subject to payment on demand.