Fortunately, a brief chat with MS Band Support led me to this elaborate but effective solution.
- Unregister the Band from within the Microsoft Health app (under My Microsoft Band in the hamburger menu [≡]).
- Remove the Band from your Bluetooth settings screen (tap and hold to Delete)
- Uninstall the Microsoft Health app
- Restart your phone (on Lumia 950 XL, press and hold power button until the phone vibrates – then release the power button and phone will re-start)
- Reinstall the Microsoft Health app
- Reset the Band (swipe to the Settings tile [⚙], swipe right to Power [⏻], swipe right to Reset Device and confirm your selection)
- Go through the Band set-up process
After this, I was able to issue voice commands to Cortana through my Band 2 again.
Update: I used the voice command feature at about 6p after resolving the issue around 1p, but by 10p when I tried to use it, the feature had failed again, displaying this message.
Update #2 (2016-04-07): I put some feelers out on common support sites for Band 2 and the Lumia 950 XL.
Cortana on Band 2 can’t reach your Phone (answers.microsoft.com)
Cortana on Band 2 can’t reach your Phone (forums.windowscentral.com)
All I was able to learn, after several weeks, is that it’s a very common problem (which is an understatement). At this time, here’s what we seem to have learned:
Band 2 Cortana works fine with iOS, Android, and Windows Phone 8.1 devices.
Band 2 Cortana doesn’t work with Windows 10 Mobile devices.
I took my fully updated Lumia 950 XL and Band 2 to my local MS store after scheduling an Answer Desk appointment. I was told that this has been a known issue since November and that there seems to be very little discussion on it since then… it seems to have stalled.
Also, since my initial post, Band 2 and Microsoft Health apps have been updated and installing these updates have not resolved the issue.
It’s troubling to me that such core functionality of flagship products has gone unrepaired for six months!
Update #3 (2016-06-09): Even though there have been several updates to Windows 10 Mobile (I’m now on Fast Ring), the Health App, and the Band 2 firmware, the problem still persists. Rod Trent over at WinSuperSite.com has written the following article about the situation though:
Constant Problems with Windows 10 and Lumia 950 Make for Irate Microsoft Band Owners