WordNet (r) 3.0 (2006):

voice mail
    n 1: a computerized system for answering and routing telephone
         calls; telephone messages can be recorded and stored and
         relayed [syn: voice mail, voicemail]

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

voice mail
phone mail

    Any system for sending, storing and
   retrieving audio messages, like a telephone answering
   machine.

   A voice mailbox is typically associated with a telephone
   number or extension.  When the number is called and the line
   is busy or not answered, the caller hears a message left by
   the owner and is given instructions for leaving a message or
   other available options, such as paging the individual or
   being transferred to an operator.  The owner of a mailbox can
   change the outgoing message or listen to incoming messages
   after entering a PIN.  Members of a voice mail system can
   generally forward or broadcast messages to other members'
   boxes.

   The experience of two people trying to reach other by
   telephone but always reaching each other's voice mail is
   referred to as "(tele)phone tag".

   (1996-11-03)