呼叫保持(Call Hold)服务使用户可暂停接收已建立的IP多媒体会话的媒体流，并在稍后恢复;

            *此多用于需与现场进行交流，同时保持电话连接，暂停通话 的场景；

1.呼叫保持业务流程

•        参与呼叫保持的分别有终端UA1,UA2和UA3，均在CSCF上完成注册

•        终端UA1完成与UA2的通话建立；
  

•        按呼叫保持(Call Hold)键，暂停与UA2的通话；

•        再按呼叫保持(Call Hold)键，恢复与UA2的通话

2.呼叫保持信令流程

3.呼叫保持主要信息：

3.1(A) INVITE sip:UA2@192.168.1.11:51948;transport=udp | , with session description

Via: SIP/2.0/UDP 192.168.1.2:64941;branch=z9hG4bK07ec3b4acc4a4392a7bedaa6532051dfcf;rport;transport=udp

Via: SIP/2.0/UDP 192.168.1.2:64869;branch=z9hG4bK81f111044c244f838fab83c8fc8fc263b01b1496;rport=64873

Max-Forwards: 69

Call-ID: 64b52f7a86a64c25ba59ac07f01604e8

CSeq: 42001 INVITE

To: <sip:UA2@Sharetechnote-cscf.com>

From: <sip:UA1@Sharetechnote-cscf.com>;tag=1111111111

Allow: INVITE, PRACK, UPDATE, ACK, BYE, CANCEL

Supported: 100rel

Supported: precondition

Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-application.ims.iari.gsma-vs";require;explicit

User-Agent: Sharetechnote-VirtualUA/3834

Content-Type: application/sdp

Content-Length: 571

Contact: <sip:UA1@192.168.1.2:64869;transport=udp>

Privacy: none

P-Asserted-Identity: <sip:UA1@Sharetechnote-cscf.com>

Record-Route: <sip:192.168.1.2;lr>

v=0

o=Sharetechnote 420 856420 IN IP4 192.168.1.2

s=-

i=A VOIP Session

c=IN IP4 192.168.1.2

t=0 0

m=audio 60000 RTP/AVP 107 97 8 0 101

b=AS:64

b=RS:800

b=RR:2400

a=ptime:20

a=maxptime:20

a=rtpmap:107 AMR-WB/16000

a=fmtp:107 octet-align=1; mode-set=2

a=rtpmap:97 AMR/8000

a=fmtp:97 octet-align=1; mode-set=7

a=rtpmap:8 PCMA/8000/1

a=rtpmap:0 PCMU/8000/1

a=rtpmap:101 telephone-event/8000

a=fmtp:101 0-15

a=mid:0

a=sendrecv

a=rtcp:60001

a=curr:qos local none

a=curr:qos remote none

a=des:qos mandatory local sendrecv

a=des:qos optional remote sendrecv

3.2(B) INVITE sip:UA2@192.168.1.11:51948;transport=udp SIP/2.0

INVITE sip:UA2@192.168.1.11:51948;transport=udp SIP/2.0

Via: SIP/2.0/UDP 192.168.1.2:64943;branch=z9hG4bK07ec3b4acc4a4392a7bedaa6532051dfd5;rport;transport=udp

Via: SIP/2.0/UDP 192.168.1.2:64869;branch=z9hG4bK81f111044c244f838fab83c8fc8fc263c24ec6e7;rport=64873

Max-Forwards: 69

Call-ID: 64b52f7a86a64c25ba59ac07f01604e8

CSeq: 42003 INVITE

To: <sip:UA2@Sharetechnote-cscf.com>;tag=340649356

From: <sip:UA1@Sharetechnote-cscf.com>;tag=1111111111

Allow: INVITE, PRACK, UPDATE, ACK, BYE, CANCEL

User-Agent: Sharetechnote-VirtualUA/3834

Content-Type: application/sdp

Content-Length: 321

Contact: <sip:UA1@192.168.1.2:64869;transport=udp>

Privacy: none

P-Asserted-Identity: <sip:UA1@Sharetechnote-cscf.com>

Record-Route: <sip:192.168.1.2;lr>

v=0

o=Sharetechnote 420 856422 IN IP4 192.168.1.2

s=-

i=A VOIP Session

c=IN IP4 192.168.1.2

t=0 0

m=audio 60000 RTP/AVP 8 0 101

b=AS:64

b=RS:800

b=RR:2400

a=ptime:20

a=maxptime:20

a=rtpmap:8 PCMA/8000/1

a=rtpmap:0 PCMU/8000/1

a=rtpmap:101 telephone-event/8000

a=fmtp:101 0-15

a=mid:0

a=sendonly

a=rtcp:60001

3.3(C) INVITE sip:UA2@192.168.1.11:51948;transport=udp SIP/2.0

Via: SIP/2.0/UDP 192.168.1.2:64945;branch=z9hG4bK07ec3b4acc4a4392a7bedaa6532051dfd9;rport;transport=udp

Via: SIP/2.0/UDP 192.168.1.2:64869;branch=z9hG4bK81f111044c244f838fab83c8fc8fc263a57af679;rport=64873

Max-Forwards: 69

Call-ID: 64b52f7a86a64c25ba59ac07f01604e8

CSeq: 42004 INVITE

To: <sip:UA2@Sharetechnote-cscf.com>;tag=340649356

From: <sip:UA1@Sharetechnote-cscf.com>;tag=1111111111

Allow: INVITE, PRACK, UPDATE, ACK, BYE, CANCEL

User-Agent: Sharetechnote-VirtualUA/3834

Content-Type: application/sdp

Content-Length: 321

Contact: <sip:UA1@192.168.1.2:64869;transport=udp>

Privacy: none

P-Asserted-Identity: <sip:UA1@Sharetechnote-cscf.com>

Record-Route: <sip:192.168.1.2;lr>

v=0

o=Sharetechnote 420 856424 IN IP4 192.168.1.2

s=-

i=A VOIP Session

c=IN IP4 192.168.1.2

t=0 0

m=audio 60000 RTP/AVP 8 0 101

b=AS:64

b=RS:800

b=RR:2400

a=ptime:20

a=maxptime:20

a=rtpmap:8 PCMA/8000/1

a=rtpmap:0 PCMU/8000/1

a=rtpmap:101 telephone-event/8000

a=fmtp:101 0-15

a=mid:0

a=sendrecv

a=rtcp:60001

原文来自：http://www.sharetechnote.com

整理：kangguoying20201019

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