FaceTime audio is a great feature in iOS 7. We use it often with our other iPhone-using friends.
It’s easy to make a FaceTime audio call. All you have to do is go to your contact and tap the phone icon under FaceTime. That’s it. You can also use Siri. Say, “Make a FaceTime Audio call to (contact name).” That’s it. “Audio” is important here, because your iPhone will make a regular FaceTime call if you don’t tell it otherwise.
The call quality is much better than that of the cellular network in our experience.