You can use FaceTime for free audio calls

Too lazy to hold your iPhone’s camera steady? Too busy to video chat? Don’t feel like installing Skype? Just use FaceTime for free, non-video, audio-only calls.

Simply start the FaceTime call like normal, but then hit the Home button to exit out to your homescreen. The call continues as voice-only, and you get the standard call bar at the top of your device to return to video if you want it.