The Collaborative International Dictionary of English v.0.48:

Telephonic \Tel`e*phon"ic\, a. [Cf. F. t['e]l['e]phonique. See
   Telephone.]
   1. Conveying sound to a great distance.
      [1913 Webster]

   2. Of or pertaining to the telephone; by the telephone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

telephonic
    adj 1: of or relating to telephony; "telephonic connection"

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "telephonic":
   Teletype, Wirephoto, communication, communicational,
   communications, facsimile, magnetotelephonic, microtelephonic,
   monotelephonic, phototelegraphic, radio, radiotelegraphic, signal,
   telecommunicational, telegraphic, telephotographic,
   thermotelephonic, wireless