[s3r3nity]

Don't work for Apple/MSFT/Google, but have knowledge of usage data for various smart-phone OS's: you'd be surprised at how _calling_ has become a minor use case across the board when it comes to smart phones.

It's now a "feature" that primarily caters to power users: if you use calling, you use it a lot - otherwise the user tends to use messaging / texting / other medium (like FaceTime.)

Therefore my design guess is that Microsoft is aiming for the video call / bluetooth headset, and will take the hit in terms of awkwardness for holding up to your ear for some calls.