There's a problem he didn't mention. It's something anyone seriously using VoIP will know - you shouldn't expect it to work on a random network - ever. Nat issues, traffic shaping, firewalls, address masking which is not like natting really, sip ipv6 support - or actually the lack of. These problems will kill your "transparent voip" experience. Of course you can setup a vpn to your server - which will cost you battery life, more firewall problems, more traffic shaping problems and delays caused by another network layer. The highly enterprisy solutions will have even more problems, like MS solutions requiring a login into the company domain. Maybe Cisco has something dedicated that would actually work...<p>In short - I think there's a long way before we can just walk into a random WiFi range and do a transparent call handover. Or you can use it on a network which has been prepared for this - but there aren't many solutions providing live call handover :( and you need to route that call through your voip to begin with (unless your mobile provider also allows live call transfers).