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Imagine you are browsing to a HTTP URL that does not exists. You'll get a 404 which often contains an explanatory body.

Now imagine you are calling someone using VoIP with SIP and use a non-existent addressee. Is it possible to return an error AND to establish an RTP stream with an explanatory audio message?

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Yes, if you look at the RFC at https://www.ietf.org/rfc/rfc3261.txt It explains exactly how to do this.

20.18 Error-Info

The Error-Info header field provides a pointer to additional information about the error status response.

  SIP UACs have user interface capabilities ranging from pop-up
  windows and audio on PC softclients to audio-only on "black"
  phones or endpoints connected via gateways.  Rather than forcing a
  server generating an error to choose between sending an error
  status code with a detailed reason phrase and playing an audio
  recording, the Error-Info header field allows both to be sent.
  The UAC then has the choice of which error indicator to render to
  the caller.

A UAC MAY treat a SIP or SIPS URI in an Error-Info header field as if it were a Contact in a redirect and generate a new INVITE, resulting in a recorded announcement session being established. A non-SIP URI MAY be rendered to the user.

Examples:

 SIP/2.0 404 The number you have dialed is not in service
 Error-Info: <sip:not-in-service-recording@atlanta.com>

The client would therefore receive the 404 in the SIP reply plus it will be told to connect to sip:not-in-service-recording@atlanta.com for a recorded message.

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