呼叫保持流程图
呼叫保持信令：
1） 用户1000发送re-INVITE消息给IPBX，须要保持1000和1002之间的通话。
Re-INVITE消息携带SDP中c=IN IP4 0.0.0.0。说明该re-INVITE消息是呼叫保持消息。
INVITE sip:1002@192.168.50.22:5060SIP/2.0
Via: SIP/2.0/UDP192.168.50.20:9588;branch=z9hG4bK-d8754z-da66ec3fc2251561-1---d8754z-;rport
Max-Forwards: 70
Contact:<sip:1000@192.168.50.20:9588>
To: <sip:1002@192.168.50.22>;tag=54267485a
From:"1000"<sip:1000@192.168.50.22>;tag=8c7df95e
Call-ID:ZjBiM2MxNDQ3ZWNlM2JhYjAwYWEyNGJlMWE4NjZjYmQ.
CSeq: 3 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS,BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO
Content-Type: application/sdp
Supported: replaces
User-Agent: Bria Professional release2.4 stamp 49381
Content-Length: 238
v=0
o=hua 9 3 IN IP4 192.168.50.20
s=CounterPath Bria Professional
c=IN IP4 0.0.0.0
t=0 0
m=audio 7624 RTP/AVP 0 101
a=fmtp:101 0-15
a=rtpmap:101 telephone-event/8000
a=sendonly
a=x-rtp-session-id:84405823042846CFA4F793EB20FEB7E6
2） IPBX收到re-INVITE请求响应，保持用户1000和1002之间的通话。而且给用户1002播放背景音乐，给用户1000回复确认消息200 OK.
SIP/2.0 200 OK
Via: SIP/2.0/UDP192.168.50.20:9588;branch=z9hG4bK-d8754z-da66ec3fc2251561-1---d8754z-;rport
From:"1000"<sip:1000@192.168.50.22>;tag=8c7df95e
To:<sip:1002@192.168.50.22>;tag=54267485a
Call-ID:ZjBiM2MxNDQ3ZWNlM2JhYjAwYWEyNGJlMWE4NjZjYmQ.
CSeq: 3 INVITE
User-Agent: FreeSwitch
Contact:<sip:1002@192.168.50.22:5060>
Content-Type: application/sdp
Content-Length: 201
v=0
o=session 1639146064 1639146065 IN IP4192.168.50.22
s=play
c=IN IP4 192.168.50.22
t=0 0
m=audio 10476 RTP/AVP 0 101
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
3） 用户1000收到IPBX发送确认请求消息200 OK，发送ACK消息给IPBX
ACK sip:1002@192.168.50.22:5060 SIP/2.0
Via: SIP/2.0/UDP192.168.50.20:9588;branch=z9hG4bK-d8754z-ef0e4b7cfe6a7548-1---d8754z-;rport
Max-Forwards: 70
Contact:<sip:1000@192.168.50.20:9588>
To:<sip:1002@192.168.50.22>;tag=54267485a
From:"1000"<sip:1000@192.168.50.22>;tag=8c7df95e
Call-ID:ZjBiM2MxNDQ3ZWNlM2JhYjAwYWEyNGJlMWE4NjZjYmQ.
CSeq: 3 ACK
User-Agent: Bria Professional release2.4 stamp 49381
Content-Length: 0