Chris Breen has a quick writeup on how to set your iPhone to route calls through your Mac (or use your Mac to make calls). Some important things to know: Your iPhone and Mac must be on the same WiFi network and you'll need iOS 8 and Yosemite installed on any target machines, plus an actual iPhone*. If you have iOS 8 on an iPad you may have already seen how you can take an incoming call via that device, but you'll need to configure your Mac to do this too. *As reader Hal Sherman points out, you'll need a Mac with native Bluetooth 4.0 LE as well -- a dongle won't do.
In FaceTime preferences on iOS and Mac, you must enable "iPhone Cellular Calls". Then you can click the Audio tab in FaceTime on the Mac to initiate calls, even to your Android-using friends. That's pretty cool, right?
While we're talking FaceTime on the Mac, did you know you can also set your Mac to route calls through Skype? The Default for calls setting allows it (at the bottom of the preferences/settings) but only on the Mac.