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VoIP NAT

506 bytes added, 15:59, 14 November 2020
Add the SIP trace of the server response
 
The bold text shows the IP address and port number that the phone is listening on - telling the far end where to send audio.
 
The server replies:
SIP/2.0 100 Trying
...
SIP/2.0 183 Session progress
From: "aaisp" <sip:+442083xxxxxx@voiceless.aa.net.uk>;tag=xidd3faqpi
To: <sip:07973xxxxxx@voiceless.aa.net.uk;user=phone>;tag=2020111412411300003
Server: FireBrick/1.54.153
Content-Type: application/sdp
o=- 31466 0 IN IP4 '''81.187.30.118'''
c=IN IP4 '''81.187.30.118'''
m=audio '''31466''' RTP/AVP 8 101
a=rtpmap:8 pcma/8000
 
So again the bold text tells the phone the IP address and port number to send the audio to.
 
The section below on "Why is SIP a problem" explains how NAT breaks things.
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