Exclude an extension number from being dialed from within the name directory

In our scenario, message-only extensions are only used to receive faxes. Patients have been able to dial random numbers while in the phone tree and leave voicemails on a specific message-only extension. These voicemails will go unanswered, as they are not even being left at a location in the same state. The ability to exclude these message-only extensions from being dialed would be ideal