IAX2 Trunk VPN Stops working

Hi All,

We have two FreePBX systems, one in Cincinnati and one in New York. There is a Fortigate VPN between the two and most functions seem to work, part of the time. The IAX2 trunk worked well for ~2 weeks and then it suddenly stopped. This has happened more than once, and to remedy the issue we had to clear some dead sessions on the router. Is there any setting on the FreePBX or router that we can look at? UDP timeouts are 60 minutes on both server and router, SIP helper and ALG are off on both routers.