Connecting a voip network to a phone network is prohibited by law. L&T paid a whooping Rs.350 Cr. as fines. So be careful with whatever it is you are planning.
You are absolutely right if you are talking about Indian context. Landing calls from VoIP to PSTN and other way around is prohibited in India. But its perfectly legal in other countries. We have no interconnection with pstn in India in picture if you read the use case again or read on how vonage works. It will be phone instrument(not connected to pstn line) <=> PAP2T like adapter <=> Appliance running skype <=> skype/landlines/cellphones. This is perfectly compliant with regulations.
thanks gs