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]