A Phantom user is simply a user account on the system that will never have a user agent (phone) registered to it. Phantoms can be used for voicemail-only mailboxes or for call routing purposes. Whatever their purpose be, phantoms should have any permissions that are not required removed for security.
A voicemail-only mailbox is useful for having callers deposit voicemail for a team of users. The voicemail should be checked on a regular basis (see Chapter 7) or have voicemail forwarded to an email address that is monitored.
Note
There is no way to set a message notification on a user's phone for anything but their own voicemail, so using email notification of messages (to an email box or cell phone) is very useful for voicemail-only mailboxes.
All permissions except voicemail permission should be removed from voicemail-only mailboxes. This prevents any SIP devices from being able to log into the proxy as the phantom user and make calls. If there are many...