It'll bounce. If it's a subscriber leaving the message it comes back to them. If it's an outside caller (i.e. an unidentified caller) it'll get bounced to the Unaddressed Messages distribution list (this is why you want an admin in that list to get these messages).
As a side note, you can configure Unity to check the inbox quota of a target subscriber before allowing unidentified callers to leave a message. There's a small delay while this happens (depending on the network/exchange responsiveness) so it's off by default. You can turn this on in the Advanced Settings Tool found in the Tools Depot on your desktop.
