Once you have iOS 7 installed, you might be surprised to see a standalone FaceTime icon on your iPhone or iPod Touch. Beyond making it clear that the device can make video calls, it also makes it easier to use FaceTime for audio.
With iOS 7, you'll be able to place audio-only calls through FaceTime, eliminating the need to watch minutes (but you'll need to monitor data usage if you're not on Wi-Fi).
The process for making an audio call via FaceTime is nearly identical to making a video call, with the lone exception being a phone icon is pressed instead of the video camera icon.
You can find this icon when viewing a contact card in the Contacts app, or in the FaceTime app itself when browsing your contact list.
Tapping on the phone will place a call, resembling a standard FaceTime video call, but once the two parties are connected, only audio will be transmitted.
When adding a contact to your favorites list, either in the FaceTime app or in the Phone app, you'll notice you can add either a FaceTime video or a FaceTime audio listing for the same user.
I've been able to use the service over a Wi-Fi and 4G connection without any major issues. At times there's a bit of lag in voice transmissions, but overall FaceTime Audio is a solid alternative to placing old-fashioned cellular calls. Of course carrier restrictions may limit who can use this feature over a data connection.