Was this PAP2 used before as a Vonage device?
You cannot use a PAP2 device that was not made for Vonage as Vonage has their own firmware in the PAP'2's that they sold. If you call to support, they can give you some options for getting another device. Vonage also keeps their ATA's, what even brand they be, user locked.
If this was a PAP2 that was used by you for Vonage, and you don't owe Vonage any money, they should let you set it up for your parents to use.