2020/05/20 11:43:13.411356 4.5.6.40:1029 -> 1.2.3.14:5060
NOTIFY sip:1.2.3.14 SIP/2.0
Via: SIP/2.0/UDP 192.168.15.177:5568;branch=z9hG4bK-7ef72342
From: "Test" <sip:test40@1.2.3.14>;tag=7b97cf77697b5442o0
To: <sip:1.2.3.14>
Call-ID: 7faf6ebb-69449dce@192.168.15.177
CSeq: 31305 NOTIFY
Max-Forwards: 70
Event: keep-alive
User-Agent: Linksys/SPA941-5.1.8
Content-Length: 0

2020/05/20 11:43:13.411521 1.2.3.14:5060 -> 4.5.6.40:5568
SIP/2.0 400 Missing Contact header
Via: SIP/2.0/UDP 192.168.15.177:5568;received=4.5.6.40;branch=z9hG4bK-7ef72342
Call-ID: 7faf6ebb-69449dce@192.168.15.177
From: "Test" <sip:test40@1.2.3.14>;tag=7b97cf77697b5442o0
To: <sip:1.2.3.14>;tag=z9hG4bK-7ef72342
CSeq: 31305 NOTIFY
Server: Asterisk PBX
Content-Length:  0