Need help, our SIP server ( Asterisk FreePBX) is on private IP and is sitting behind the fortigate firewall at our head office all local extension are working fine. We have one branch office and want to register phone from this office on SIP server which is sitting on head office.
We configures IPSec tunnel, SIP server is pinging and telnet to port 5060 is also happening but extension registration failing. 408 request time out.
Looks like network connection issue. try to make PCAP on Far End Side.
Some Phones have inside Web Interface PCAP options. I think PCAP will give you more information from which side 408 message you are receiving.
Check the logs and to a packet capture and see what address the phone is connecting from. Once you know what the PBX Is seeing for addresses, you should add that address to the local network list in the PJ-SIP settings.