某厂家的话机和IPBX进行对接问题：
1. 该话机作为主叫方，呼叫能够正常建立
2. 该话机作为被叫方。呼叫无法建立，IPBX发送INVITE消息给该话机，该话机回复400
具体消息例如以下：
IPBX->P1
INVITEsip:302@172.16.251.157 SIP/2.0
Via:SIP/2.0/UDP 172.16.250.127:5060;branch=4694cb8a
From:"301" <sip:301@172.16.250.127>;tag=288bc0ba
Contact:<sip:301@172.16.250.127:5060>
To:<sip:302@172.16.251.157>
Call-ID:14391754550e6c0a8658dc0efb4864da89@7ffa10ac
CSeq:2200 INVITE
Max-Forwards:70
Content-Type:application/sdp
Content-Length:319
v=0
o=root1184156554 1184156554 IN IP4 172.16.250.127
s=session
c=IN IP4172.16.250.127
t=0 0
m=audio25118 RTP/AVP 0 8 18 101
a=rtpmap:0PCMU/8000
a=rtpmap:8PCMA/8000
a=rtpmap:18G729/8000
a=rtpmap:101telephone-event/8000
a=fmtp:1010-15
P1->IPBX
SIP/2.0 400 Bad Request
Via: SIP/2.0/UDP 172.16.250.127:5060;branch=4694cb8a
From: "301"<sip:301@172.16.250.127>;tag=288bc0ba
Contact:<sip:301@172.16.250.127:5060>
To: <sip:302@172.16.251.157>
Call-ID:14391754550e6c0a8658dc0efb4864da89@7ffa10ac
CSeq: 2200 INVITE
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 319
v=0
o=root 1184156554 1184156554 IN IP4172.16.250.127
s=session
c=IN IP4 172.16.250.127
t=0 0
m=audio 25118 RTP/AVP 0 8 18 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:18 G729/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
依据如上消息进行分析branch=4694cb8a没有添加z9hG4bK值（该值的添加设置到事物的处理详见事物处理），因此推断IPBX支持的是RFC2543协议，话机支持的是RFC3261协议。
具体处理过程例如以下：
1. 依据上述的分析，查看话机是否支持RFC2543协议
2. 查看话机配置页，发现有RFC2543和RFC3261协议选项，因此，选择RFC2543重新启动话机
3. 话机重新启动后。发现作为被叫呼叫还是建立不起来
4. 在IPBX上的branch简单添加z9hG4bK值后，呼叫正常建立
5. 从以上分析发现，该厂商的话机仅仅是简单的推断branch中是否存在z9hG4bK值，没有严格的依照RFC3261协议中的事物来进行处理
6. 到此。和该话机对接成功。成功的消息详见以下
IPBX->P1
INVITE sip:8501@172.16.251.157:5060 SIP/2.0
Via: SIP/2.0/UDP172.16.250.191:5060;branch=z9hG4bK1c1ac64a;rport
Max-Forwards: 70
From: " 8500"<sip:8500@172.16.250.191>;tag=as76dfef91
To: <sip:8501@172.16.251.157:5060>
Contact:<sip:8500@172.16.250.191:5060>
Call-ID: 4b99a0636512f93430c4e9dd58fa9832@172.16.250.191:5060
CSeq: 102 INVITE
Content-Type: application/sdp
Content-Length: 233
v=0
o=root 628132733 628132733 IN IP4172.16.250.191
s=BPBX 2.0.0.0
c=IN IP4 172.16.250.191
t=0 0
m=audio 18088 RTP/AVP 8 101
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
P1->IPBX
SIP/2.0 200 OK
Via: SIP/2.0/UDP172.16.250.191:5060;branch=z9hG4bK1c1ac64a;rport
From: " 8500"<sip:8500@172.16.250.191>;tag=as76dfef91
To:<sip:8501@172.16.251.157:5060>;tag=169336615
Call-ID: 4b99a0636512f93430c4e9dd58fa9832@172.16.250.191:5060
CSeq: 102 INVITE
Contact:<sip:8501@172.16.251.157:5060>
Content-Type: application/sdp
Content-Length: 217
v=0
o=802 30033847 10069422 IN IP4172.16.251.157
s=AC
c=IN IP4 172.16.251.157
t=0 0
m=audio 10028 RTP/AVP 8 101
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=ptime:20
a=sendrecv