It's a dead simple service — combine a SIP URI with a click to call application, and now you can call anyone in the world. I popped my Truphone SIP URI into the appplication, and was receiving calls in seconds on my mobile phone.
Voxalot's pitch is that VoxCall is self-organizing and enables anonymity. If your VoxCall friend changes his contact phone number, you don't have to be notified. VoxCall will use whatever number your friend has registered so you no longer need to remember his number, just his Facebook ID. In addition, VoxCall also offers both public and private chat rooms where VoxCall friends can congregate.
While it works well, there's a lot of click-to-call competition out there. Perhaps the biggest differentiator for Voxcall is simply that it hooks into a directory that a lot of people know and use.