You can try to use 'tcpdump -s 0 ...' to see what comes there. Beware that tcp is a stream protocol and network sniffing tools can see parts of the same messages coming as separate packet on the wire, which can be a matter of how the kernel flushes the outbuffer.

Also, don't forget about the option with onsend_route to see what kamailio is writting to the socket.

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