由于用戶的流動性，經常會出現移出網絡服務范圍；此時做為被叫就會出現“您所撥叫的用戶不在服務區(qū)”的情況，針對此情況IMS/SIP在3GPP TS.24.604中定義了用戶可達的前轉（ Communication Forwarding on Subscriber Not Reachable --CFNRc) ；
* 這種情況一般是由于終端周期性更新超時或移出服務區(qū)，核心網側隱含關機時發(fā)生；
1.終端注冊，參與用戶不可達前轉(UA)需要：
A.參與終端（UA1,UA2,UA3)在CSCF進行注冊；
B.并不可達前轉條件；
2.不可達前轉信令流程
3.不可達前轉信令主要消息內容
3.1(A) INVITE sip:UA2@ims.mnc246.mcc081.3gppnetwork.org SIP/2.0
Via: SIP/2.0/UDP [2001:0:0:1::2]:56439;branch=z9hG4bK1161335930;rport
From: <sip:UA1@test.3gpp.com>;tag=1161341877
To: <sip:UA2@ims.mnc246.mcc081.3gppnetwork.org>
Contact: <sip:UA1@[2001:0:0:1::2]:56439;transport=udp>;+g.oma.sip-im;language="en,fr";+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel"
Call-ID: bdc0cdba-3278-1303-7151-eada26d4de3a
CSeq: 21140 INVITE
Content-Type: application/sdp
Content-Length: 739
Max-Forwards: 70
Route: <sip:[2001:0:0:1::2]:5060;lr;transport=udp>
Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel"
P-Preferred-Service: urn:urn-7:3gpp-service.ims.icsi.mmtel
Allow: INVITE, ACK, CANCEL, BYE, MESSAGE, OPTIONS, NOTIFY, PRACK, UPDATE, REFER
Privacy: none
P-Access-Network-Info: ADSL;utran-cell-id-3gpp=00000000
User-Agent: IM-client/OMA1.0 Boghe/v2.0.112.744
P-Preferred-Identity: <sip:123456789@test.3gpp.com>
Supported: 100rel
v=0
o=doubango 1983 678901 IN IP6 2001:0:0:1::2
s=-
c=IN IP6 2001:0:0:1::2
t=0 0
m=audio 8048 RTP/AVP 112 111 3 8 0 9 97 98 99 101
a=ptime:20
a=rtpmap:112 AMR/8000/1
a=imageattr:112 octet-align=1
a=fmtp:112 octet-align=1
a=rtpmap:111 AMR/8000/1
a=imageattr:111 octet-align=0
a=fmtp:111 octet-align=0
a=rtpmap:3 GSM/8000/1
a=rtpmap:8 PCMA/8000/1
a=rtpmap:0 PCMU/8000/1
a=rtpmap:9 G722/8000/1
a=rtpmap:97 SPEEX/8000/1
a=rtpmap:98 SPEEX/16000/1
a=rtpmap:99 SPEEX/32000/1
a=rtpmap:101 telephone-event/8000/1
a=fmtp:101 0-15
a=tcap:1 RTP/AVPF
a=pcfg:1 t=1
a=sendrecv
a=rtcp-mux
a=ssrc:1311262921 cname:ldjWoB60jbyQlR6e
a=ssrc:1311262921 mslabel:6994f7d1-6ce9-4fbd-acfd-84e5131ca2e2
a=ssrc:1311262921 label:Doubango
3.2 (B) 181 Call Is Being Forwarded
Max-Forwards: 70
Via: SIP/2.0/UDP [2001:0:0:1::2]:56439;branch=z9hG4bK1161335930;rport=56439
From: <sip:UA1@test.3gpp.com>;tag=1161341877
To: <sip:UA2@ims.mnc246.mcc081.3gppnetwork.org>;tag=987654321
Call-ID: bdc0cdba-3278-1303-7151-eada26d4de3a
CSeq: 21140 INVITE
History-Info: <sip:001010123456789@ims.mnc246.mcc081.3gppnetwork.org>; index=1
Content-Length: 0
3.3(C) INVITE sip:UA3@[2001:0:0:1::2]:49329;transport=udp;cause=503 SIP/2.0
Via: SIP/2.0/UDP [2001:0:0:1::2]:28101;branch=z9hG4bKbf2ece8e7f3e42539582fdba3875fe9d198;rport;transport=udp
Via: SIP/2.0/UDP [2001:0:0:1::2]:56439;branch=z9hG4bK1161335930;rport=56439
From: <sip:UA1@test.3gpp.com>;tag=1161341877
To: <sip:UA3@ims.mnc246.mcc081.3gppnetwork.org>
Contact: <sip:UA1@[2001:0:0:1::2]:56439;transport=udp>;+g.oma.sip-im;language="en,fr";+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel"
Call-ID: bdc0cdba-3278-1303-7151-eada26d4de3a
CSeq: 21140 INVITE
Content-Type: application/sdp
Content-Length: 739
Max-Forwards: 69
Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel"
P-Preferred-Service: urn:urn-7:3gpp-service.ims.icsi.mmtel
Allow: INVITE, ACK, CANCEL, BYE, MESSAGE, OPTIONS, NOTIFY, PRACK, UPDATE, REFER
Privacy: none
P-Access-Network-Info: ADSL;utran-cell-id-3gpp=00000000
User-Agent: IM-client/OMA1.0 Boghe/v2.0.112.744
Supported: 100rel
P-Asserted-Identity: <sip:123456789@test.3gpp.com>
History-Info: <sip:UA2@ims.mnc246.mcc081.3gppnetwork.org>; index=1
History-Info: <sip:UA3@ims.mnc246.mcc081.3gppnetwork.org?cause=503>; index=1.1; MP=1
Record-Route: <sip:[2001:0:0:1::2]:28101;lr>
v=0
o=doubango 1983 678901 IN IP6 2001:0:0:1::2
s=-
c=IN IP6 2001:0:0:1::2
t=0 0
m=audio 8048 RTP/AVP 112 111 3 8 0 9 97 98 99 101
a=ptime:20
a=rtpmap:112 AMR/8000/1
a=imageattr:112 octet-align=1
a=fmtp:112 octet-align=1
a=rtpmap:111 AMR/8000/1
a=imageattr:111 octet-align=0
a=fmtp:111 octet-align=0
a=rtpmap:3 GSM/8000/1
a=rtpmap:8 PCMA/8000/1
a=rtpmap:0 PCMU/8000/1
a=rtpmap:9 G722/8000/1
a=rtpmap:97 SPEEX/8000/1
a=rtpmap:98 SPEEX/16000/1
a=rtpmap:99 SPEEX/32000/1
a=rtpmap:101 telephone-event/8000/1
a=fmtp:101 0-15
a=tcap:1 RTP/AVPF
a=pcfg:1 t=1
a=sendrecv
a=rtcp-mux
a=ssrc:1311262921 cname:ldjWoB60jbyQlR6e
a=ssrc:1311262921 mslabel:6994f7d1-6ce9-4fbd-acfd-84e5131ca2e2
a=ssrc:1311262921 label:Doubango
原文來自：http://www.sharetechnote.com
整理：kangguoying20201017
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