WordNet (r) 3.0 (2006):

call on
    v 1: have recourse to or make an appeal or request for help or
         information to; "She called on her Representative to help
         her"; "She turned to her relatives for help" [syn: call
         on, turn]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "call on":
   adjure, advocate, appeal, appeal to, apply to, beg, beseech, bid,
   blandish, cajole, call for help, call the signals, call upon,
   charge, circulate a petition, clamor for, coax, command,
   commission, conjure, crave, cry for, cry on, cry to, declare,
   decree, dictate, direct, drop in, enjoin, entreat, exhort,
   give an order, give the word, high-pressure, impetrate, implore,
   importune, imprecate, insist, insist upon, instruct, invoke,
   issue a command, issue a writ, jawbone, kneel to, lobby, look in,
   mandate, memorialize, nag, obtest, ordain, order, order about,
   pay a visit, petition, plead, plead for, plead with, pray,
   prefer a petition, press, pressure, proclaim, promulgate,
   pronounce, push, recommend, rule, run in, run to, say the word,
   see, sign a petition, soft-soap, stop by, stop off, stop over, sue,
   supplicate, sweet-talk, urge, visit, wheedle, work on