The Collaborative International Dictionary of English v.0.48:

sign on \sign` on"\ (s[imac]n` [o^]n"), v. i.
   To begin a communication, especially one conducted by radio
   waves.
   [PJC] Signor

WordNet (r) 3.0 (2006):

sign on
    v 1: engage by written agreement; "They signed two new pitchers
         for the next season" [syn: sign, contract, sign on,
         sign up]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "sign on":
   Teletype, affiliate, affiliate with, associate, beam, bespeak,
   book, brief, broadcast, cable, call up, combine, come into,
   commandeer, conscript, creep in, detach, detach for service, draft,
   employ, engage, enlist, enroll, enter, flash, get into, go into,
   hire, impress, inaugurate, induct, initiate, install, instate,
   invest, join, join up, league with, levy, list, mobilize, muster,
   muster in, newscast, ordain, preengage, press, radiate, radio,
   radiobroadcast, raise, recruit, reserve, retain, send, send a wire,
   shortwave, sign off, sign up, sign up for, sneak in, sportscast,
   summon, take into employment, take on, take out membership,
   take up membership, team up with, team with, telegram, telegraph,
   telex, transmit, wire, wireless