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

Voice over IP
VoIP

    (VoIP) Any technology providing voice
   telephony services over IP, including CODECs, streaming
   protocols and session control.  The major advantage of
   VoIP is lower cost, by avoiding dedicated voice circuits.

   Currently VoIP is being deployed on internal corporate
   networks, and, via the Internet, for low cost (and low
   quality) international calls.  It is also used for telephony
   applications such as voice and fax mail.

   The ITU standard is H.323, which is a whole suite of
   protocols, while the IETF has developed the much simpler
   SIP to solve the session control problem and MGCP/Megaco
   to solve the gateway problem.

   (2003-11-30)