FortiVoice delivers integrated calling, conferencing, chat and fax.
This article explains how to disable a voicemail box on a user extension.

The extension profile settings do not have any quick toggle for the voicemail on each extension because this is the default action for all of the normal call handling options.

In order to disable voicemail on user extensions a profile should be created containing this restriction.  In early builds voicemail could not be disabled, but it has been added as an option.

Click on Call Features > User Privileges

Since the default profile has it enabled, a new one must be required in order to disable voicemail on only some of the extensions. After creating a new profile, select the required options and also uncheck the 'Voicemail Enabled' box. Select 'OK', and return to the phone list of users.

For any extension where voicemail is not required change their User privilege box to the new profile that has been created and save.