Apple is bringing support for Continuity over Cellular with iOS 9 and is already working with T-Mobile to make the feature available to developers on iOS 9 beta.
Continuity lets you receive and make calls on your Mac and iPad via your iPhone. Previously the feature required your devices to be on the same Wi-Fi network; however, today T-Mobile revealed that Continuity support has been added to its cellular network, letting a Mac or iPad receive an iPhone's call even if it isn't on the same Wi-Fi network.
"T-Mobile customers will be able to answer that important text message or call on your Mac or iPad even if you left your phone at home," says T-Mobile, so "you can leave your phone on your desk and just take your tablet or your Mac to your meeting and never worry about missing anything."
"Customers will need the iOS 9 beta to use the new feature, and it will be available to every T-Mobile customer with an iOS device later this year when iOS 9 is publicly available."
The settings in iOS 9 have been updated to reflect the feature's new capabilities. There's a switch for "Calls on Other Devices" and the ability to individually toggle which devices to allow calls on.
T-Mobile notes that "while others will get on the bandwagon", it's "proud to be the only mobile operator in the world working with Apple to have the new Continuity feature on our network in the iOS 9 beta."
I don't like this feature as I get my wife's calls and messages and she gets mine, also my moms and brothers and everyone else. I have like 10 people using my Apple ID for their phones that way the my can use everything I have purchased, but I hate getting all of their messages and calls. We all have to turn of iMessage and continuity or else we would all go crazy.
Apple now has family sharing. You can add your family member to family sharing and they can maintain their own account while being able to share purchased items. You should look into it. It's very useful.
sounds like they're allowing you to connect to their network using a virtual sim card that will link all your devices. (thats the best explanation I could come up with as far as how it will work in the backend)