Apple introduced Continuity features with iOS 8 last year, basically it’s a set of features that allows iOS devices to work seamlessly between them and with Macs. Users could even receive text messages and phone calls on their Macs provided that the iPhone and the Mac were on the same Wi-Fi network. iOS 9 does away with this requirement as it’s capable of supporting Continuity phone calls over cellular connections.
iOS 9 is going to make it much easier for users to receive and respond to text messages and phone calls on Macs or iPads, given that they don’t need all of the devices to be on the same Wi-Fi network as the iPhone.
It will work off of the iPhone’s cellular connection, this means that the handset can even be in an entirely different location and yet be able to route text messages and phone calls as long as the Mac or iPad is connected to the internet.
This means that if a user were to forget their iPhone at home they would be able to take all of the calls throughout the day without having to leave the office.
However this features requires Wi-Fi calling to be enabled and T-Mobile is the first carrier in the country to support it. This feature is not yet available on iPhones that run on other networks but who knows, maybe they’ll also flip the switch on this feature by the time iOS 9 is publicly released this fall.