Enabling VoIP support from the GUI
Before you begin to configure VoIP security profiles, including SIP, from the GUI you should go to System > Feature Select and turn on VoIP (under Additional Features).
Also, VoIP security profiles are only available if the FortiGate or current VDOM Inspection Mode is set to Proxy-based in the System Information Dashboard widget. If the Inspection mode is set to Flow-based the inspection mode will not be visible from the System Information Dashboard widget. You must use the following CLI command to change the Inspection mode to proxy:
config system settings
set inspection-mode proxy