Here is a letter I wrote to Apple Accessibility and their quick response.
I'm writing you with a very strange problem or at least that's the way it seems to me.
I'm using an iPhone 6S with iOS 9.3.2
I have my notifications and alerts configured to speak with Voice Over even when the screen is locked.
Everything works fine with the phone when I have a belt clip on in this case Mopie's belt clip with an extended Battery.
But, when I use the pouch or holster, I hear only the sound for the alert but not Voice Over reading the alert.
When I take it out of the holster, all works as it should.
I thought this was a mophie problem until tonight when I had the phone in my sweat pants pocket while working out.
Again alerts were not read. Its like voice over is being blocked. But, I don't see how that could be happening.
When I emailed a knowledgable friend, "Thomas donville" he wrote:
My first thought that came across my mind is that when the phone thinks it
is face down. It will make sounds but VoiceOver will not talk. I believe
this is some sensor that detects when it is face down to save on battery.
I'm not 100% on this but I wonder if the sensor thinks it is face down? This
would be a good question to take to the Apple Store.
By the way, the phone is not face down in the Mophie holster or in my sweat pants pocket.
I've had 4S and 5S phones but don't remember this being a problem.
I even tried this test using a friends 6S phone and her Mophie holster with the same results.
If you want to test this behavior, just make sure the apps on your phone have been configured so the alerts read when the screen is locked.
I would be interested in what you discover.
Apple's response on Friday June 10 2016
Thank you for your email. What you are reporting is being caused by the ambient light sensor being covered while in the pouch. The same behavior will occur if the iPhone is face down on a table. We are passing a suggestion regarding this behavior along to the appropriate team on your behalf.