I doubt there is a technical approach to this - but I really like the simultaneous ring feature - however; if one of the 2nd or 3rd phones is a cell phone - and one of them is off - the voice mail on that cell will pick up right away - and only that cell phone user will know of the message.
It would be way cooler to force any voice mail to the Obi or something.
Since we live out in the land of hit and miss cell towers - the cell phones would be trying to capture voice mails.