I'll give it a shot and get back to you. I figured from the logic in the voip-ui that I would need to extend the dbus logic with another callback, but I'll see how it gets passed down.

Time to get hacking. The nice thing here is that this is a clean way to handle it. If I write a dialing pad for the home screen I can just do a urlopen and pass it sip: or tel: and if the hook is not there it will open the built-in voip client.

But I ramble.