The latest beta version of what would become the second major update to iOS 9, which was seeded to developers yesterday, enables Wi-Fi Calling through AT&T on your Mac, 9to5Mac discovered.
While Wi-Fi Calling allows you to place a phone call in an area with little or no cellular coverage, bringing AT&T Wi-Fi Calling to the Mac and other devices you own lets you make and receive phone calls on your Mac (or iPad or iPod touch, for that matter) without needing to be on the same network, or even in the same area, as your iPhone.