Starting in iOS 7 you could already make audio-only Facetime calls person-to-person. But with the upcoming point release of Max OS X Mavericks, you’ll be able to place audio calls from your Mac as well.
Check out 9 to 5 Mac for the details.
You may recall that Facetime uses extremely high-quality audio for voice calls–much better than you can get over traditional phone lines. This will be big.