What is "on call"

Call \Call\, n.

1. The act of calling; -- usually with the voice, but often otherwise, as by signs, the sound of some instrument, or by writing; a summons; an entreaty; an invitation; as, a call for help; the bugle's call. ``Call of the trumpet.''
   --Shak.

   I rose as at thy call, but found thee not.
   --Milton.

2. A signal, as on a drum, bugle, trumpet, or pipe, to summon soldiers or sailors to duty.

3. (Eccl.) An invitation to take charge of or serve a church as its pastor.

4. A requirement or appeal arising from the circumstances of the case; a moral requirement or appeal.

   Dependence is a perpetual call upon humanity.
   --Addison.

   Running into danger without any call of duty.
   --Macaulay.

5. A divine vocation or summons.

   St. Paul himself believed he did well, and that he had a call to it, when he persecuted the Christians.
   --Locke.

6. Vocation; employment.

   Note: [In this sense, calling is generally used.]

7. A short visit; as, to make a call on a neighbor; also, the daily coming of a tradesman to solicit orders.

   The baker's punctual call.
   --Cowper.

8. (Hunting) A note blown on the horn to encourage the hounds.

9. (Naut.) A whistle or pipe, used by the boatswain and his mate, to summon the sailors to duty.

10. (Fowling) The cry of a bird; also a noise or cry in imitation of a bird; or a pipe to call birds by imitating their note or cry.

11. (Amer. Land Law) A reference to, or statement of, an object, course, distance, or other matter of description in a survey or grant requiring or calling for a corresponding object, etc., on the land.

12. The privilege to demand the delivery of stock, grain, or any commodity, at a fixed, price, at or within a certain time agreed on. [Brokers' Cant]

13. See Assessment, 4. At call, or On call, liable to be demanded at any moment without previous notice; as money on deposit. Call bird, a bird taught to allure others into a snare. Call boy

    1. A boy who calls the actors in a theater; a boy who transmits the orders of the captain of a vessel to the engineer, helmsman, etc.

    2. A waiting boy who answers a cal, or cames at the ringing of a bell; a bell boy.

       Call note, the note naturally used by the male bird to call the female. It is artificially applied by birdcatchers as a decoy.
       --Latham.

       Call of the house (Legislative Bodies), a calling over the names of members, to discover who is absent, or for other purposes; a calling of names with a view to obtaining the ayes and noes from the persons named.

       Call to the bar, admission to practice in the courts.

alt. available when requested. prep.phr. available when requested.

On call may refer to:

• On Call, a 1974 novel by Elizabeth Harrison (writer)
• " On Call", a 2007 song by Kings of Leon
• On Call: Serbisyo. Publiko. Ngayon., a Philippine news and public service program
• On-call room, an area of a hospital or police station
• DART On-Call, a premium shuttle service by Dallas Area Rapid Transit
• being available for work on request while off-duty.