Forwarded on No Answer
Here User A attempts to call User B, who does not answer. The call is forwarded to User B's mailbox, and the voicemail system plays a message for a ring-no-answer. The call flow then moves from User A to the voicemail system.
F1
INVITE sip:UserB@wcom.com SIP/2.0
Via: SIP/2.0/UDP here.com:5060
From: TheBigGuy
To: TheLittleGuy
Call-Id: 12345600@here.com
CSeq: 1 INVITE
Contact: TheBigGuy
Proxy-Authorization:Digest username="UserA",
realm="MCI WorldCom SIP",
nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",
uri="sip:UserB@wcom.com", response=
calculated hash goes here>
Content-Type: application/sdp
Content-Length:
v=0
o=UserA 2890844526 2890844526 IN IP4 client.here.com
s=Session SDP
c=IN IP4 100.101.102.103
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
Here B1 rings for, let's say, nine seconds. (This duration is a configurable parameter in the Proxy Server.) The Proxy sends a Cancel option and proceeds down its internal list of options, eventually selecting a voicemail URL for "forward no answer."
SIP/2.0 180 Ringing
F5 Via: SIP/2.0/UDP here.com:5060From: TheBigGuy
To: TheLittleGuy ;tag=3145678
Call-Id: 12345600@here.com
CSeq: 1 INVITE
Content-Length: 0
Here the voicemail system is ready to set up a RTP session, and record the message by using the 200 OK signal. The 'contact' field indicates where the file is being stored.
F9 SIP/2.0 200 OK
Via: SIP/2.0/UDP wcom.com:5060; branch=2
Via: SIP/2.0/UDP here.com:5060
Record-Route:
From: TheBigGuy
To: TheLittleGuy ;tag=123456
Call-Id: 12345600@here.com
CSeq: 1 INVITE
Contact: TheLittleGuyVoiceMail
Content-Type: application/sdp
Content-Length:
v=0
o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
s=Session SDP
c=IN IP4 110.111.112.114
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
Finally User A hangs up on the voicemail system ...but the VM system could have disconnected the call on its own.
F13 BYE sip:UserB@wcom.com SIP/2.0
Via: SIP/2.0/UDP here.com:5060
Route:
From: TheBigGuy
To: TheLittleGuy ;tag=123456
Call-Id: 12345600@here.com
CSeq: 2 BYE
Content-Length: 0
