iOS 8 introduced Continuity, which allowed Mac and iOS devices to interact in new ways, including the ability for both the Mac and iPad to receive text messages and phone calls if they, along with your iPhone, were all on the same Wi-Fi network. iOS 9 extends that, by allowing call forwarding over a cellular connection.
That means an iPhone can continue to forward calls to a Mac or an iPad even when it’s in an entirely different physical location. So, for example, if you forget your iPhone at home, you can continue to receive incoming calls on your Mac while at work, so long as your Mac is connected to a Wi-Fi network.
The feature is part of the Wi-Fi calling feature on the iPhone. As such, T-Mobile is the first U.S. carrier to offer support for the cellular Continuity feature. T-Mobile devices running iOS 9 include settings that can be toggled to allow calls on other devices.
“Wi-Fi calling for other devices allows other devices signed into your iCloud account to make and receive calls using your carrier account even when your iPhone is not nearby,” reads the description of the feature.
If you’re on an iPhone from another carrier, you’ll see a different message: “Use your iPhone cellular connection to make and receive calls on devices signed into your iCloud account when they are nearby and on Wi-Fi.”
If you’re a developer running the iOS 9 beta on your T-Mobile iPhone, you can use this feature immediately. Public beta testers will be able to try it out in July, when iOS 9 is released for a public beta. No word on when other carriers will support the feature.