在日常通話中你可能會(huì)遇到被叫正在通話或呼叫的情況，而遇忙前轉(zhuǎn)（Communication Forwarding on Busy user -CFB)就是針對(duì)這種“對(duì)方（因忙）無（法）應(yīng)答”做出r將呼叫轉(zhuǎn)移到第三方（接受-接聽）處理；3GPP在TS 24.604對(duì)“對(duì)方無應(yīng)答前轉(zhuǎn)“Communication forwarding on no reply”有具體定義；
1. 遇忙前轉(zhuǎn)(CFB)流程
支持遇忙前轉(zhuǎn)的相關(guān)網(wǎng)元，首先需要完成在CSCF中的注冊(cè)，然后確定前轉(zhuǎn)的規(guī)則（具體到那個(gè)UA);
2. 遇忙前轉(zhuǎn)(CFB)主要消息
2.1（A) INVITE sip:UA1@ims.mnc246.mcc081.3gppnetwork.org SIP/2.0
Via: SIP/2.0/TCP [2001:0:0:1::2]:58110;branch=z9hG4bK680245c87bed4ef089641f3f7cc62263f580f1da;transport=tcp
Max-Forwards: 70
Call-ID: c55a5d495a874c14bc886ec31edf3eef
CSeq: 18001 INVITE
To: sip:UA1@ims.mnc246.mcc081.3gppnetwork.org
From: sip:UA2@test.3gpp.com;tag=2222222222
Feature-Caps: *;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call
Allow: ACK, BYE, CANCEL, INVITE, PRACK, UPDATE
Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";require;explicit
Privacy: none
User-Agent: Sharetechnote-VirtualUA/7e501f1
Content-Type: application/sdp
Content-Length: 727
Route: <sip:[2001:0:0:1::2]:28101;lr>
Contact: <sip:UA3@[2001:0:0:1::2]:58110;transport=tcp>;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";video;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call
v=0
o=Sharetechnote 599 64599 IN IP6 2001:0:0:1::2
s=-
i=A VOIP Session
c=IN IP6 2001:0:0:1::2
t=0 0
m=audio 60002 RTP/AVP 96 97 98 99 100 8 0 101 102
b=AS:153
b=RS:800
b=RR:2400
a=ptime:20
a=maxptime:240
a=rtpmap:96 EVS/16000/1
a=fmtp:96 max-red=220
a=rtpmap:97 AMR-WB/16000/1
a=fmtp:97 mode-change-capability=2
a=rtpmap:98 AMR-WB/16000/1
a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 AMR/8000/1
a=fmtp:99 mode-change-capability=2
a=rtpmap:100 AMR/8000/1
a=fmtp:100 octet-align=1; mode-change-capability=2
a=rtpmap:8 PCMA/8000/1
a=rtpmap:0 PCMU/8000/1
a=rtpmap:101 telephone-event/16000
a=fmtp:101 0-15
a=rtpmap:102 telephone-event/8000
a=fmtp:102 0-15
a=mid:0
a=rtcp:60003
a=sendrecv
2.2（C) SIP/2.0 486 Busy Here
Via: SIP/2.0/UDP [2001:0:0:1::2]:5060;branch=z9hG4bK73cbbac348344ef799e1ace1e004354486;rport;transport=udp
Via: SIP/2.0/TCP [2001:0:0:1::2]:58110;branch=z9hG4bK680245c87bed4ef089641f3f7cc62263f580f1da;transport=tcp
Max-Forwards: 70
Record-Route: <sip:[2001:0:0:1::2]:5060;lr>
Contact: <sip:001010123456789@[2001:0:0:1::11]:5060>;+sip.instance="<urn:gsma:imei:35975507-290776-0>";+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";+g.3gpp.mid-call;+g.3gpp.srvcc-alerting;+g.3gpp.ps2cs-srvcc-orig-pre-alerting;video
To: <sip:UA1@ims.mnc246.mcc081.3gppnetwork.org>;tag=7ce82475
From: <sip:UA3@test.3gpp.com>;tag=2222222222
Call-ID: c55a5d495a874c14bc886ec31edf3eef
CSeq: 18001 INVITE
Supported: timer, 100rel
User-Agent: T-Mobile VoLTE-ePDG-IR94 SEC/G930TUVU4Z 7.0
P-Access-Network-Info: 3GPP-E-UTRAN-FDD;utran-cell-id-3gpp=0010100010000000
Content-Length: 0
2.3 (D) SIP/2.0 181 Call Is Being Forwarded
Max-Forwards: 70
Via: SIP/2.0/TCP [2001:0:0:1::2]:58110;branch=z9hG4bK680245c87bed4ef089641f3f7cc62263f580f1da;transport=tcp
From: sip:UA3@test.3gpp.com;tag=2222222222
To: sip:UA1@ims.mnc246.mcc081.3gppnetwork.org;tag=7ce82475
Call-ID: c55a5d495a874c14bc886ec31edf3eef
CSeq: 18001 INVITE
History-Info: <sip:001010123456789@ims.mnc246.mcc081.3gppnetwork.org>; index=1
History-Info: <sip:001010123456789@ims.mnc246.mcc081.3gppnetwork.org>; index=1.1; NP=1
Content-Length: 0
2.4 (E) INVITE sip:UA4@[2001:0:0:1::2]:55899;transport=udp;cause=486 SIP/2.0
Via: SIP/2.0/UDP [2001:0:0:1::2]:5060;branch=z9hG4bK73cbbac348344ef799e1ace1e00435448b;rport;transport=udp
Via: SIP/2.0/TCP [2001:0:0:1::2]:58110;branch=z9hG4bK680245c87bed4ef089641f3f7cc62263f580f1da;transport=tcp
Max-Forwards: 69
Call-ID: c55a5d495a874c14bc886ec31edf3eef
CSeq: 18001 INVITE
To: sip:UA4@test.3gpp.com
From: sip:UA3@test.3gpp.com;tag=2222222222
Feature-Caps: *;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call
Allow: ACK, BYE, CANCEL, INVITE, PRACK, UPDATE
Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";require;explicit
Privacy: none
User-Agent: Sharetechnote-VirtualUA/7e501f1
Content-Type: application/sdp
Content-Length: 727
Contact: <sip:UA3@[2001:0:0:1::2]:58110;transport=tcp>;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";video;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call
P-Asserted-Identity: <sip:VirtualUA_2nd@test.3gpp.com>
History-Info: <sip:001010123456789@ims.mnc246.mcc081.3gppnetwork.org>; index=1
History-Info: <sip:001010123456789@ims.mnc246.mcc081.3gppnetwork.org>; index=1.1; NP=1
History-Info: <sip:123456789@test.3gpp.com?cause=486>; index=1.2; MP=1.1
Record-Route: <sip:[2001:0:0:1::2]:5060;lr>
v=0
o=Sharetechnote 599 64599 IN IP6 2001:0:0:1::2
s=-
i=A VOIP Session
c=IN IP6 2001:0:0:1::2
t=0 0
m=audio 60002 RTP/AVP 96 97 98 99 100 8 0 101 102
b=AS:153
b=RS:800
b=RR:2400
a=ptime:20
a=maxptime:240
a=rtpmap:96 EVS/16000/1
a=fmtp:96 max-red=220
a=rtpmap:97 AMR-WB/16000/1
a=fmtp:97 mode-change-capability=2
a=rtpmap:98 AMR-WB/16000/1
a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 AMR/8000/1
a=fmtp:99 mode-change-capability=2
a=rtpmap:100 AMR/8000/1
a=fmtp:100 octet-align=1; mode-change-capability=2
a=rtpmap:8 PCMA/8000/1
a=rtpmap:0 PCMU/8000/1
a=rtpmap:101 telephone-event/16000
a=fmtp:101 0-15
a=rtpmap:102 telephone-event/8000
a=fmtp:102 0-15
a=mid:0
a=rtcp:60003
a=sendrecv
原文來自：http://www.sharetechnote.com
整理：kangguoying20201015
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