Almost exactly last year, Apple revealed the Continuity feature of the then upcoming OS X Yosemite, a feature that, among other things, allowed Mac users to make and take calls on their Macs as long as their iPhone or iPad is within range. Useful as that may be, there was one subset of users that were left out in the cold. Those on T-Mobile that used Wi-Fi calling couldn’t get their hands on Continuity. That is, until later this year when iOS 9 rolls out with that particular feature along for the ride.
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https://www.slashgear.com/ios-9-continuity-to-support-t-mobile-wi-fi-calls-11387864/