GUI-VOIP-Profile not available in every VDOM

hello,

We would like to activate gui-voip-profile to see voip profiles in the GUI.

Some VDOM's have this already working with the following command:

! config vdom edit vdom_name config system settings set gui-voip-profile enable !

If i try to add it on other VDOMs i don't see the option:

Firewall (settings) # set gui-voip-profile enable

command parse error before 'gui-voip-profile' Command fail. Return code -61

We use the following fortigate:

Version: FortiGate-1000C v5.6.11,build1700,190814 (GA)