Merlin Messaging, or Merlin Mail?
Will the extension still have a mailbox?
Do you want callers in the AA to be denied the transfer, or bypass ringing the phone and going directly to the mailbox?
Merlin Messaging, you can NOT create a mailbox, and NOT give it transfer-only permissions, and the transfer will be denied. Of course there is also no mailbox for the extension.
Merlin Messaging, you can create the mailbox, but set the extension as "Without a Phone", transfers from the AA will go directly to the msilbox. The extension will NOT get a message waiting indication, because there's no reason to try and light a light for an extension that supposedly doesn't exist.
Merlin Mail, mailbox class of service 7 does not allow transfer (like without a phone in Merlin Messaging),