Call Forward On Busy

In this scenario User B wants calls forwarded to another destination if the original line is busy. It is assumed that the proxy knows where to forward the call.

F1 INVITE A -> Proxy
INVITE sip:UserB@ss1.wcom.com SIP/2.0
Via: SIP/2.0/UDP here.com:5060
From: BigGuy
To: LittleGuy
Call-ID: 12345600@here.com
CSeq: 1 INVITE
Contact: BigGuy
Proxy-Authorization: DIGEST username="UserA", realm="MCI WorldCom
SIP", nonce="9137d175c20a0d6eadd7be1c863302ae", opaque="",
uri="sip:ss1.wcom.com", response="cf25aad811c806bde46a369220158cec"
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

F4 486 Busy Here B1 -> Proxy

User B's phone responds back with a busy message ( 486 )

SIP/2.0 486 Busy Here
Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=1
Via: SIP/2.0/UDP here.com:5060
From: BigGuy
To: LittleGuy ;tag=123456
Call-ID: 12345600@here.com
CSeq: 1 INVITE
Content-Length: 0

F6 INVITE Proxy -> B2

The call is then forwarded to a new location at "THERE.COM." Since the call is going to a new location, a new INVITE and session description is sent.

INVITE sip:UserB2@ there.com SIP/2.0
Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=2
Via: SIP/2.0/UDP here.com:5060
Record-Route:
From: BigGuy
To: LittleGuy
Call-ID: 12345600@here.com
CSeq: 1 INVITE
Contact: BigGuy
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