<p>Maybe you can attach a pcap with the traffic for this case, from the REGISTER sent to Kamailio to the INVITE exposing the issue.</p>
<p>There are some aspects related to SIP that you confuse or maybe you didn't explain it properly above. The Contact header in the INVITE is not the socket of Kamailio because Kamailio is not generating the INVITE. If the device is behind the NAT, the sip server cannot connect to it, unless the nat router is a port forwarding firewall.</p>
<p>Kamailio is reusing the tcp connection whenever the destination address is matching an active one. The pcap with the sip traffic should clarify what happens.</p>

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