If it was used with vonage recently, you’re locked out. At the request of large service providers like Vonage they added a feature to the firmware to password protect the factory reset command. You’d have to ask Vonage for the password – which I suspect would be something like trying to get a cellphone unlocked.
I feel your pain – I’d planned other uses for my Vonage ATA at the end of this year and now it’ll be a paperweight.