The Collaborative International Dictionary of English v.0.48:

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.
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            I rose as at thy call, but found thee not. --Milton.
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   2. A signal, as on a drum, bugle, trumpet, or pipe, to summon
      soldiers or sailors to duty.
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   3. (Eccl.) An invitation to take charge of or serve a church
      as its pastor.
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   4. A requirement or appeal arising from the circumstances of
      the case; a moral requirement or appeal.
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            Dependence is a perpetual call upon humanity.
                                                  --Addison.
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            Running into danger without any call of duty.
                                                  --Macaulay.
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   5. A divine vocation or summons.
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            St. Paul himself believed he did well, and that he
            had a call to it, when he persecuted the Christians.
                                                  --Locke.
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   6. Vocation; employment.

   Note: [In this sense, calling is generally used.]
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   7. A short visit; as, to make a call on a neighbor; also, the
      daily coming of a tradesman to solicit orders.
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            The baker's punctual call.            --Cowper.
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   8. (Hunting) A note blown on the horn to encourage the
      hounds.
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   9. (Naut.) A whistle or pipe, used by the boatswain and his
      mate, to summon the sailors to duty.
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   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.
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   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.
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   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]
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   13. See Assessment, 4.
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   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
       (a) 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.
       (b) 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.
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Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "on call":
   COD, accessible, adaptable, all-around, at demand, at hand,
   available, cash, cash down, cash on delivery, convenient, down,
   handy, money down, of all work, on deck, on demand, on hand,
   on tap, pay-as-you-go, ready, strictly cash, to hand,
   upon presentation, versatile