AT&T's VoWiFi is identical. I can call from my MacBook with my iPhone on airplane mode.mavericks and later Yosemite and iOS 8 requires the iPhone to be on the same wifi network and powered on to place and receive calls and to send and receive regular text messages.
t-mobile (and soon sprint), El Capitan and IOS 9 no longer requires the phone to on the same wifi network. it does not even require the phone to be on. and it does not even need the iPhone to be within range of any of your devices.
hopefully , continuity including "calls on other devices" over cellular will come to AT&T as well