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Recently, i had to troubleshoot an Asterisk to Asterisk trunk which was running across a site to site IPSec VPN. (Fortinet to Cisco)
After running tcpdump “port 5060 and proto UDP” on either end, I discovered traffic from the Cisco end was not reaching the PBX behind the Fortinet. Packet capture on the Fortinet showed traffic being matched, and classified as SIP.
We had done the usual commands to stop the Fortigate from acting as a SIP ALG, but nothing was working. After a bit of tinkering, i found that the following command fixed our problem. (Basically fooled the Fortigate into thinking SIP traffic, was not SIP)

config system settings
 set sip-udp-port 5067