The only way that I know is to prevent these users from dialing off-net for Message Notification, via the COS and Restriction Tables related to this COS. Have a look;
Restriction tables allow you to control which phone numbers subscribers and administrators can use for:
Transferring calls including both the numbers subscribers can enter for transferring their calls, and the numbers that unidentified callers can enter when using Caller system transfers.
Recording and playback by phone from Cisco Unity applications, when the phone is the designated recording and playback device in the Media Master. (The Media Master is available in the Cisco Unity Administrator, the Cisco Unity Assistant, the Cisco Unity Inbox, and ViewMail.)
Delivering faxes to a fax machine.
***Sending message notifications.
Sending AMIS messages.
For example, you can specify that subscribers have calls transferred only to internal extensions or that faxes are delivered only to local phone numbers. Restriction tables are applied regardless of how a subscriber or administrator accesses Cisco Unity.
Each class of service specifies for its members a restriction table for call transfers, ***one for message notification, and one for fax deliveries. The restriction table can be the same for all three, or different for each. Note, however, that the AMIS restriction table is not associated with a class of service; there is only one systemwide Restriction Table that controls AMIS message delivery.
Select a Restriction Table Select a restriction table for each of the following:
Outcalling Select a restriction table to limit phone numbers that subscribers assigned to this COS can enter in message delivery settings. The table you select also restricts the subscriber extensions that Cisco Unity dials when the phone is selected as the recording and playback device for the Media Master.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.