According to their website, "FriendCaller works instantly with every popular operating system and browser without installation. And, of course with the iPhone™ and iPod™ touch." It then says, "Send your friends call-me links directly from your iPhone™ or iPod™ touch. FriendCaller is instant browser based Voice, your friends don't have to install software or register with FriendCaller before talking to you through their browsers. They just click-to-call and you are connected and see each other online status."
So it sounds like you have to send people you want to call a special hyperlink, which they have to click to then initiate an outbound call back to you. I'm guessing they embed your iPhone's regular GSM phone number into the click-to-call link which gets passed to the Java applet. Then the person you are trying to reach clicks the click-to-call link launching the Java app on their browser and automatically initiating an outbound call to your iPhone. The call is routed as voice over IP from the remote caller's PC/Mac to C2Call's termination network (for low cost VoIP routing) and then terminates to your iPhone. This is speculation on my part based on the info I read and the technical limitations of the iPhone (no Java), but it's the only explanation I can come up with on how it works.
So it isn't truly end-to-end VoIP and it isn't even an outbound VoIP call. It's more like an inbound (PC/Mac/Linux) VoIP-to-GSM/PSTN call.
Update: See the comments for more technical details
Anywhere, here's the features:
- Free calls to anybody on the Internet over WiFi
- Instant messaging and Call-me links in WiFi and mobile data
- Full access to your iPhone™ contacts from FriendCaller
- Friend List with online status
- Low cost calls to phone numbers including mobiles over WiFi
- Call-me links work instantly on Windows XP/Vista, MAC OS-X and Linux