What is Continuity? It is Apple’s protocol that allows you to take what you are doing from 1 device and continue on another. You could be composing a mail message on you mac and finish it on your iPhone. Another example is being able to take a cellular call from your iPhone and answer it from anywhere on your iPad, iPod Touch, or Mac.
This is very cool especially if you left your phone at home and you have your iPad with you, T-Mobile will let you answer that call as long as you have a Wifi or cellular connection. You can use Continuity to make and respond to text messages and iMessages as well. Currently iOS 9 is still in beta, but when it is available to the general public this will be a standard feature for everyone on T-Mobile.