在日常通话中你可能会遇到被叫正在通话或呼叫的情况，而遇忙前转（Communication Forwarding on Busy user -CFB)就是针对这种“对方（因忙）无（法）应答”做出r将呼叫转移到第三方（接受-接听）处理；3GPP在TS 24.604对“对方无应答前转“Communication forwarding on no reply”有具体定义；

1. 遇忙前转(CFB)流程

       支持遇忙前转的相关网元，首先需要完成在CSCF中的注册，然后确定前转的规则（具体到那个UA);

2. 遇忙前转(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

原文来自：http://www.sharetechnote.com

整理：kangguoying20201015

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