The OS X Mavericks 10.9.2 update added Facetime Audio to the Mac. Here's how you can use it to make audio calls with your Facetime friends and family.
1. Open the FaceTime app on your Mac.
2. Select the contact you want to call.
3. Click the FaceTime Audio button.
If your contact is on a Mac or iOS device your call will go through and you can talk via audio.
Some folks might wonder why somebody would want to use only the audio portion of FaceTime. There are times when video is unwanted and unnecessary (like when you first get up in the morning and it looks like a nuclear bomb went off in your hair), so being able to make audio-only Facetime calls can be incredibly convenient.