My understanding of what the parent wrote is the phone line would be disconnected in the normal case - can't break out if it's physically disconnected.
I wonder… wouldn't a physically disconnected ethernet cable work as well?
That design probably has more with phone network penetration rather than corporate firewalls. The most likely use case for on prem Google would be far away sites with no connectivity except a phone line
We used it for intranet documents but I suppose it had multiple uses.
The phone line is disconnected, but maybe some other link isn't.
I mean it would be an extremely cool hack but I wouldn't be super-surprised to learn that google deployed some sort of exfiltration search technology in these boxes.