The causeparameters can indicate to the caller or to a service component
where to redirect the call and what operations to perform next.
For example, a call that has been redirected will now send an INVITEsuch as:INVITE sip:[email protected];\
target=sip:+15555551002%40example.com;user=phone;\
cause=486 SIP/2.0
Via: SIP/2.0/TCP 192.0.2.4:5060;branch=z9hG4bK-ik80k7g-2
Via: SIP/2.0/TCP 192.0.2.1:5060;branch=z9hG4bK-74bf9
From: Alice <sip:[email protected];user=phone>;tag=9fxced76sl
To: sip:[email protected];user=phone
Call-ID: c3x842276298220188511
CSeq: 1 INVITE
Max-Forwards: 70
Contact: <sip:[email protected]>
Content-Type: application/sdp
Content-Length: *Body length goes here*...* SDP goes here*The Causein line three is 486 and indicates the reason is User Busy.Voicemail
Users can control services either by voice or by using forms on Web servers.
We will show in this example how a user can invoke voicemail using the Web
server. The simplified call flows are shown in Figure 19.3.
As shown in Figure 19.4, the caller uses a Web page to click on the URL of
the called party intended to receive the voice message. The Web server
requests in Message 1 the controller to connect the caller with the voicemail
server. The controller then connects the user’s SIP client with the voicemail
server using SIP third-party call control. The call to the message server is
accepted in Message 7, and the SDP data from the message server is conveyed
in the re-INVITEin Message 8, giving the SIP client the necessary information
where to send the audio.328 Chapter 19