FortiOS has two features that can modify the SIP headers and SDP parameters. The first feature is called the “SIP Session Helper”. If you are experiencing one way audio issues disable this feature first, reboot your IP phone then try making another call. If disabling the session helper does not work, disable the SIP ALG as well.
To disable the sip session helper:
1 Enter the following command to find the sip session helper entry in the session-helper list:
show system session-helper
set name sip
set port 5060
set protocol 17
2 Enter the following command to delete session-helper list entry number 10 to disable the sip session helper:
config system session-helper
To disable the SIP ALG:
There are typically two VOIP profiles on a factory shipped Fortinet firewall. You may need to disable both profiles to fully stop the ALG.
config voip profile
set status disable
See the Fortigate Technical documentation page for further details.