The Jargon File (version 4.4.7, 29 Dec 2003):

voice-net
 n.

    Hackish way of referring to the telephone system, analogizing it to a
    digital network. Usenet sig blocks not uncommonly include the sender's
    phone next to a ?Voice:? or ?Voice-Net:? header; common variants of this
    are ?Voicenet? and ?V-Net?. Compare paper-net, snail-mail.


The Free On-line Dictionary of Computing (30 December 2018):

voice-net

   Hackish way of referring to the plain old telephone system,
   comparing it to a digital network.  Usenet sig blocks
   sometimes include the sender's telephone number next to a
   "Voice:" or "Voice-Net:" header; variants of this are
   "Voicenet" and "V-Net".

   Compare paper-net, snail-mail.

   [Jargon File]

   (1995-02-16)