该项服务允许一个用户对于他的呼入呼叫可以转到另一个号码。使用该业务时对该用户号码的所有呼叫，不管被叫用户是在什么状态，都自动转到一个预先指定的号码。
例如：用户3000设置无条件前传3001；用户2000呼叫3000，此时3001振铃，显示主叫号码2000。详细消息流程如下：
A (2000) IPBX C(3001)
|----invite(3000)-->------|------invite(3001)--->-----|
|----<--200OK -------------|----<--200OK---------------|
|----------ACK--->----------|-----------ACK-->------------|
|------------RTP-------------|------------RTP---------------|
(1) A(2000)呼叫B(3000)
(2) 用户A的invite请求到底IPBX，IPBX对被叫号码3000进行分析
(3) 被叫号码3000设置无条件前传号码3001
(4) IPBX转发invite请求给用户3001
(5) 被叫3001摘机，并回复200OK消息给IPBX
(6) IPBX收到200OK，并转发给主叫方
(7) 主叫方收到200OK，回复确认消息ACK
(8) IPBX转发ACK确认消息给用户C
主叫方2000呼叫用户3000
INVITEsip:3000@172.161.10.8:5060;transport=udp SIP/2.0
Via: SIP/2.0/UDP172.161.10.79:5060;rport;branch=z9hG4bK664046211
From: "2000"<sip:2000@172.161.10.8:5060;user=phone>;tag=1696473636
To: <sip:3000@172.161.10.8>
Call-ID: 742673760@172.161.10.79:5060
CSeq: 38 INVITE
Contact:<sip:2000@172.161.10.79:5060>
Max-Forwards: 30
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS,NOTIFY, REFER, REGISTER, MESSAGE
Content-Type: application/sdp
Content-Length: 295
v=0
o=ower 1376039693 1376039693 IN IP4172.161.10.79
s= ower
c=IN IP4 172.161.10.79
t=0 0
m=audio 62830 RTP/AVP 0 8 18 3 4 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:18 G729/8000
a=rtpmap:3 GSM/8000
a=rtpmap:4 G723/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
IPBX收到主叫方的呼叫请求，并分析被叫号码3000，发现被叫码号3000设置无条件前传号码3001；IPBX发送呼叫请求给被叫号码3001
INVITE sip:3001@172.161.10.3 SIP/2.0
Via: SIP/2.0/UDP172.161.10.8:5060;branch=z9hG4bK77f8c784
Supported: 100rel
Allow: INVITE, ACK, CANCEL, BYE, UPDATE,REFER, PRACK, REGISTER, INFO
From: "2000"<sip:2000@172.161.10.3>;tag=0b2a58bb
Contact: <sip:2000@172.161.10.8:5060>
To: <sip:3001@172.161.10.3>
Call-ID:1376039727002cc015416297db@172.161.10.8
CSeq: 1435 INVITE
Max-Forwards: 70
Supported: timer
Session-Expires: 90
Content-Type: application/sdp
Content-Length: 308
v=0
o=root 2012792708 2012792709 IN IP4172.161.10.8
s=session
c=IN IP4 172.161.10.8
t=0 0
m=audio 15920 RTP/AVP 0 8 18 3 4 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:18 G729/8000
a=rtpmap:3 GSM/8000
a=rtpmap:4 G723/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=ptime:10
3001收到IPBX发送的呼叫请求，摘机后向IPBX发送200OK
SIP/2.0 200 OK
Via: SIP/2.0/UDP172.161.10.8:5060;branch=z9hG4bK77f8c784
From: "2000"<sip:2000@172.161.10.3>;tag=0b2a58bb
To:<sip:3001@172.161.10.3>;tag=310e43c7c
Call-ID:1376039727002cc015416297db@172.161.10.8
CSeq: 1435 INVITE
Contact: <sip:3001@172.161.10.3:5060>
Content-Type: application/sdp
Content-Length: 199
v=0
o=root 1426816092 1426816092 IN IP4172.161.10.3
s=session
c=IN IP4 172.161.10.3
t=0 0
m=audio 15530 RTP/AVP 0 101
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
IPBX收到被叫方发送的摘机响应后，并转发200OK响应给主叫方
SIP/2.0 200 OK
Via: SIP/2.0/UDP172.161.10.79:5060;rport;branch=z9hG4bK664046211
From: "2000"<sip:2000@172.161.10.8:5060;user=phone>;tag=1696473636
To:<sip:3000@172.161.10.8>;tag=7e5f4f914
Call-ID: 742673760@172.161.10.79:5060
CSeq: 38 INVITE
Contact: <sip:3000@172.161.10.8:5060>
Content-Type: application/sdp
Content-Length: 211
v=0
o=root 1410984694 1410984694 IN IP4172.161.10.8
s=session
c=IN IP4 172.161.10.8
t=0 0
m=audio 10276 RTP/AVP 0 101
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=ptime:10
主叫方2000收到IPBX发送的摘机响应后，回复确认消息ACK给IPBX
ACK sip:3000@172.161.10.8:5060 SIP/2.0
Via: SIP/2.0/UDP172.161.10.79:5060;branch=z9hG4bK57319558
From: "2000"<sip:80001@172.161.10.8:5060;user=phone>;tag=1696473636
To: <sip:3000@172.161.10.8>;tag=7e5f4f914
Call-ID: 742673760@172.161.10.79:5060
CSeq: 38 ACK
Max-Forwards: 30
Content-Length: 0
IPBX收到主叫方发送的摘机确认消息ACK，并转发给被叫方，至此通话建立成功，可以进行语音通话
ACK sip:3001@172.161.10.3:5060 SIP/2.0
Via: SIP/2.0/UDP172.161.10.8:5060;branch=z9hG4bK77f8c784
From: "2000"<sip:2000@172.161.10.3>;tag=0b2a58bb
To: <sip:3001@172.161.10.3>;tag=310e43c7c
Call-ID:1376039727002cc015416297db@172.161.10.8
CSeq: 1435 ACK
Max-Forwards: 70
Content-Length: 0
原文地址：http://blog.csdn.net/wh8_2011/article/details/45842153
