ceott,
That is because there is an Extension ID of that phone already so that the VM knows it exists. If you go to Extension IDs instead of Mailboxes and delete the Extension ID it will then allow you to create an Associated Mailbox.
Of course you will still need to create a forwarding path for the phone so it know when to go to its mailbox, but looking at a phone that already has it should allow you do that pretty easily; if not just let us know. You can also do a Copy/Paste from one phone to another, just uncheck everything but System Forwarding and then hit ok and only the system forwarding path will be copied over.
Thanks,
TE