log in :tail -f /var/log/message:
Sep 4 05:18:50 appliance /usr/local/sbin/kamailio[3117]: ERROR: tls [tls_server.c:392]: SSL error:error:14094418:SSL routines:SSL3_READ_BYTES:tlsv1 alert unknown ca
in portgo : certificate validation failure.
It is rather clear - your SIP client does not accept the proxy's certificate and thus terminates the TLS handshake with an "unknown ca" error.
You have to configure your SIP client to accept the CA which has signed the proxy's certificate.
regards klaus