Project XIII is now available for the iPhone in addition to the Windows phone! You can download it via the iTunes App store. You will need to tell others your Id to talk to friends. But that's just a matter of an email or telephone call.
The app is free for 60 days, and 99 cents a month thereafter to cover network expenses and new development. I purchased 12 months of subscription, then added another 12 months to that. The more cash that flows into Caleb the sooner we get voice over IP functionality like Skype has. I'd really like to dump Skype soon.
This is a really neat little app, seems to work well, but this is the first release and there's more functionality to come in subsequent updates.
PS: It also seems to work just fine on the iPad according to reports.