WordNet (r) 3.0 (2006):

listen in
    v 1: listen quietly, without contributing to the conversation
    2: listen without the speaker's knowledge; "the jealous man was
       eavesdropping on his wife's conversations" [syn: listen in,
       eavesdrop]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "listen in":
   attend, attend to, auscultate, be all ears, bend an ear, bug, buzz,
   call, call up, cock the ears, dial, eavesdrop, examine by ear,
   give a ring, give attention, give audience to, give ear, hang up,
   hark, hear, hear out, hearken, heed, hold the phone, intercept,
   lend an ear, listen, listen at, listen to, make a call, phone,
   ring, ring off, ring up, sit in on, tap, telephone, tune down,
   tune in, tune up, wiretap