Vonage says they'll refund the $50 termination fee if you get an RMA and return the box. I'm cancelling my Vonage service this week so we'll see if I can give them back their ATA. FYI, the phone number isn't part of the device's fixed configuration so if someone else wanted to use it with a new Vonage account, they could.
Presumably because he hopes he can find suckers who don't realize that they're locked to Vonage. Or maybe he's so dim he doesn't realize it.