As far as I know, FaceTime audio only allows you to call other FaceTime users, not old-fashioned landlines, mobile phones etc.
Google Voice is nothing new though. Google voice already offered VOIP (I regularly use an app called "Talkatone" for this). What's changed is that Google now offers a nicely packaged, official app for this.
FaceTime only lets you connect to that small percentage of the world that uses FaceTime. FaceTime won't work on Windows (91% of PCs) nor will it work on Android (80% of shipped smartphones last quarter). So it's not really a viable solution unless you only talk to Apple users.
No snark intended. Just included some stats for context. I'm not sure where you are, but I have a few dev friends in the valley that have a habit of forgetting that context.
