Log into UCP, and view a voicemail in the VM widget. You can use the check toggle on the left to select a message and use the Forward button at the top to send the message to another VM box. This works as expected.
But if you use the Forward arrow button on the right, this one:
you get different behavior. Instead of a freeform field to enter a destination VM box, you get a drop down menu with a partial selection of mailboxes.