Skype uses a proprietary protocol to call other Skype users and provides SkypeOut to access PSTN services.
Vonage on the other hand is a service provider that provides VoIP capability by using open standards such as SIP and IAX.
That said though, Vonage is still a walled garden in that they do not allow inbound access from other SIP providers. This in my opinion makes them no different to Skype.
I have found that the best solution to VoIP is to sign up to any SIP provider that allows other network inbounds and use a service like SIP Broker http://www.sipbroker.com/ to bridge these networks via peering and ENUM.
Slashdot Mirror
Judging by the latest sneak peek photo from NASA, it looks like money well spent.
Vonage on the other hand is a service provider that provides VoIP capability by using open standards such as SIP and IAX.
That said though, Vonage is still a walled garden in that they do not allow inbound access from other SIP providers. This in my opinion makes them no different to Skype.
I have found that the best solution to VoIP is to sign up to any SIP provider that allows other network inbounds and use a service like SIP Broker http://www.sipbroker.com/ to bridge these networks via peering and ENUM.