Continuity is one of the best things about going all in on Apple hardware. With it, you can easily send and receive calls or text messages from your Mac or iPad — even when your iPhone is in another room. But one requirement of using Continuity is that both devices must be on the same Wi-Fi network. With iOS 9, that's set to change, and T-Mobile wants the world to know it's the first major US carrier to offer Continuity through its cellular network.
Note: Contrary to what the article implies, making and receiving phone calls is only part of Continuity.