In the Partner world, you would create your guest mailboxes within the extension range of the Partner, 10 through 57. You would also have to turn on Auto VMS Coverage (#310) for each "extension" and set the VMS Coverage rings (#321) to 1.
In the Legend/Magix world, every physical digital phone (MLX and TDL) has a 2nd associated extension, called an adjunct. On an MLX, you can physically install a circuit board (adjunct) in the bottom of the phone, to give you an independent single line phone, or alert. TDL's don't have this physical possibility, but the adjunct extension numbers are still there in the programming. Since no one really ever uses these physical adjuncts, their extension numbers become available for "phantom" extensions.
You can't dial an extension or send a call to a mailbox if you can't dial the extension number from another phone. So if you make a mailbox number in the Merlin Messaging that doesn't match an extension number somewhere in the system, you can't get there from here.
So, print your Extension Directory. You'll see an extension number in the 7300 range associated with each digital extension number.
Pick one, go to Renumber > Single > Adjunct > Dial the Old Adjunct Extension Number > Enter > Dial the phantom Extension Number that matches your mailbox > Enter
Go to Extensions > More > Group Coverage > 30 (usually) > Enter > Dial the phantom Extension Number > Enter
Now you will be able to dial the extension number from a phone, it will ring, and cover to the mailbox.