On-call is a term used to describe a situation where a person is available to respond to an emergency or urgent request at any time. There are various synonyms that can be used to describe this concept. Some of these terms include standby, available, vigilant, responsive, and ready. Each of these words conveys a similar meaning to on-call, emphasizing the need to be prepared and at the ready to respond to a sudden need. Other synonyms for on-call include accessible, present, at-the-ready, on-hand, and on-demand. These words all express the need for an individual to be available and responsive to any emergency situation or urgent request that may arise.