Disregard pls, it was my mistake that I didn't publish the Kamailio certs on FreeSWITCH so the initial handshake failed and Kamailio kind of fell back to try to send it via UDP. Best Regards Dimitry Nagorny Trainee Von: sr-users [mailto:sr-users-bounces@lists.sip-router.org] Im Auftrag von Nagorny, Dimitry Gesendet: Dienstag, 26. April 2016 09:49 An: Kamailio (SER) - Users Mailing List &lt;sr-users(a)lists.sip-router.org&gt; Betreff: [SR-Users] force_send_socket Hi all, I've configured Kamailio to use TLS and it has two NICs, 192.x.x.x extern to a PSTN and 10.250.5.74 intern to FreeSWITCH. Kamailio has "listen=tls:10.250.5.74:5071" and later on in the config I fire "force_send_socket(tls:10.250.5.74:5071)". For some reason on calls from FreeSWITCH via Kamailkio to PSTN the TLS works but the other way around if Kamailio should do TLS to FreeSWITCH it throws this warning: "/kamailio[3308]: WARNING: <core> [forward.c:231]: get_send_socket2(): protocol/port mismatch (forced tls:10.250.5.74:5071, to udp:10.250.5.72:5071)" As a result of this Kamailio tries to send the PSTN INVITE via UDP and also tries to use the external IP (192.x.x.x) and of course this can't work. Why is it throwing this warning and how to understand it? Best regards Dimitry Nagorny Trainee