Hi Licky

You can start here.

https://community.fortinet.com/t5/FortiGate/Technical-Tip-VoIP-and-SIP-configuration-and-troubleshooting/ta-p/342841

For me it was a long journey (a couple years ago) but all I remember is that I could make it work by enabling proxy-based inspection in the related firewall rule (I forgot other details).

as Avaya uses lots of ports for communication so thats why I need to apply IPS and application filter, when i do apply them, especially IPS, communication goes haywire, I have only blocked the higher 3 severities in IPS but still something stucks and sometimes can't find any clue from logs as there wasn't any explicit denial.