Actually Vonage uses a protocol similar to SIP to setup calls to the adapter in your home. On the other end is a software based phone switch and a gateway which coverts VoIP to regular phone service. That is connected to a normal phone company such as Verizon or SBC or in the case of Libretel, British Telecom. Microsoft Windows Messenger is a SIP client, but it needs a gateway to call a normal telephone. Libretel seems to be providing such a gateway. But they can also layer on things like 911, LNP etc. It seems their first twist is getting virtual numbers in places that the others haven't tried yet.