As long the person you are calling is a registered Gizmo user, you can call any of their lines (PSTN, mobile, Gizmo, etc) for FREE in 60 countries worldwide. Gizmo works just like Skype and actually works better than Skype through your VPN du jour.
The new offer of course, comes with free PC-to-PC calling, which is nothing special, and quite common place. It also allows free calling between companies that use the Asterisk PBX software. There a lot of companies selling pre-configured Asterisk PBXs including Digium which launched an Asterisk appliance last week. SIPphone makes money by selling long distance minutes, and is betting that if more people switch to these open source VoIP platforms, they will use it as a way to reach the PSTN/Mobile phones.