Thanks to a change that eliminates the need to be on the same Wi-Fi network to use Continuity, it looks like users will be able to make and receive calls and texts on their Mac or iPad over a cellular connection with iOS 9. This will require the support of carriers, and T-Mobile is confirming that the feature is currently working with its network in the iOS 9 beta. From The Verge:
...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. Your iPhone won't need any Wi-Fi connection to pass off calls and text conversations to your Mac or iPad, though obviously the latter two devices will still need to be online for this to work.
With the ability to respond to calls and texts even if you leave your phone at home, one can't deny that this is a bit of a game-changer for Continuity's usefulness. While T-Mobile is the first to roll out support for the feature (while iOS 9 is in beta form, no less), it's hard to imagine that the other carriers are far behind. It's unclear if they may add support during the beta, or sometime after iOS 9 goes live.
Source: The Verge