[SR-Users] Understanding port changing

[Social Boh]

Hello list,

I'm looking for to understand a problem have with Kamailio.

The configuration is:

user (behind NAT) -> Kamilio (Public IP) -> user (behind NAT)

When I make a call from user A to user B (both registered) , User B 
receive the INVITE correctly but in the ringing SIP message change the 
port present on the Contact Header and same think on 200OK so when 
Kamailio try to send it ACK SIP message to this port fail because the 
Softphone listen on the port where has received the INVITE from Kamailio.

This happens with 3 different Softphone: BRIA, Xlite, Linphone on the 
same computer.

Thank you

Regards

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I'm SoCIaL, MayBe