1. Introduction

   For some forms of communication, people prefer to communicate using
   their voices rather than typing.  By permitting voicemail to be
   implemented in an interoperable way on top of Internet Mail, voice
   messaging and electronic mail no longer need to remain in separate,
   isolated worlds, and users will be able to choose the most
   appropriate form of communication.  This will also enable new types
   of devices, without keyboards, to be used to participate in
   electronic messaging when mobile, in a hostile environment, or in
   spite of disabilities.

   There exist unified messaging systems that will transmit voicemail
   messages over the Internet using SMTP/MIME, but these systems suffer
   from a lack of interoperability because various aspects of such a
   message have not hitherto been standardized.  In addition, voicemail
   systems can now conform to the Voice Profile for Internet Messaging

   (VPIM v2 as defined in RFC 2421(-> 3801draft) [VPIMV2] and revised in RFC 3801draft,
   Draft Standard [VPIMV2R2]) when forwarding messages to remote
   voicemail systems.  VPIM v2 was designed to allow two voicemail
   systems to exchange messages, not to allow a voicemail system to
   interoperate with a desktop e-mail client.  It is often not
   reasonable to expect a VPIM v2 message to be usable by an e-mail
   recipient.  The result is messages that cannot be processed by the
   recipient (e.g., because of the encoding used), or look ugly to the
   user.

   This document therefore proposes a standard mechanism for
   representing a voicemail message within SMTP/MIME, and a standard
   encoding for the audio content, which unified messaging systems and
   mail clients MUST implement to ensure interoperability.  By using a
   standard SMTP/MIME representation and a widely implemented audio
   encoding, this will also permit most users of e-mail clients not
   specifically implementing the standard to still access the voicemail
   messages.  In addition, this document describes features an e-mail
   client SHOULD implement to allow recipients to display voicemail
   messages in a more friendly, context-sensitive way to the user, and
   intelligently provide some of the additional functionality typically
   found in voicemail systems (such as responding with a voice message
   instead of e-mail).  Finally, how a client MAY provide a level of
   interoperability with VPIM v2 is explained.

   It is desirable that unified messaging mail clients also be able to
   fully interoperate with voicemail servers.  This is possible today,
   providing the client implements VPIM v2 [VPIMV2R2], in addition to
   this specification, and uses it to construct messages to be sent to a
   voicemail server.

   The definition in this document is based on the IVM Requirements
   document [GOALS].  It references separate work on critical content
   [CRITICAL] and message context [HINT].  Addressing and directory
   issues are discussed in related documents [ADDRESS], [VPIMENUM],
   [SCHEMA].

   Further information on VPIM and related activities can be found at
   http://www.vpim.org or http://www.ema.org/vpim.