Disable the SIP ALG/Session Helper on the Fortigate

Last update: 30.10.2025

General

Your VoIP provider should give you the information, if the SIP ALG on the Fortigate is needed or not.

In the default setting of a Fortigate the SIP ALG is active.

Possible issues

If a VoIP provider does not require a SIP ALG/Session Helper on the Gateway but the SIP ALG/Session Helper is still active, it can among others cause the following issues:

• One-way voice
• Telephones not ringing
• Issues with the registration of the Telephone can occur
• Call transferring might fail
• Calls that are on hold cannot be taken over

Solution

If you encounter any of the above issues, try to deactivate the SIP ALG and the session helper on the Fortigate. The commands are as follows:

# Disable status and rtp in the default voip profile
config voip profile
edit default
config sip
set status disable
set rtp disable
end
end

# Show the ID of the SIP session helper
sh | grep "name sip" -f
# If some output is shown, use the ID to delete the entry
config system session-helper
delete <id>
end

The above changes will only take effect after the Fortigate has been rebooted. To reboot the Fortigate issue the command below:

execute reboot

With the following command you can check if a session helper is active on a session currently open in your FortiGates session table:

diag sys session list | grep sip