Vonage's consumer plans with unlimited calling use their own, locked-down ATAs. They don't give out the SIP credentials. This is to prevent abuse of their service, by users provisioning that unlimited account on multiple devices or in multiple locations.
You can simply port the numbers out of Vonage, to a SIP VoIP Internet Telephone Service Provider (ITSP) that does offer open credentials. For example, Callcentric will port your numbers in for free, and, being an OBiTALK-supported service provider, it just takes a few minutes to set it up on your OBi phone(s).