When receiving a phone call, the iPhone will occasionally become unresponsive. VoiceOver does not speak, and you cannot navigate around the screen to see who is calling or choose any options. This has happened when the phone is both muted or unmuted, and often happens to one user when using bluetooth audio.
Happens to me (iPhone 6 Plus) almost the 100% of times tried if I press power button after the first ring tone, while listening any audio through EarPods
Maybe this helps to find a reliable pattern
I have been able to reproduce it in six devices: all of them Iphone 5s or 6.
1. Plug in your headphones via the minijack port, turn on VoiceOver and make sure your iphone is not in silence mode so that the ring tone can be heard when you receive an incoming call.
2. Manage to have an incoming call placed to your iphone so that the ringtone starts playing. You might notice how the volume of your ringtone goes up and down.
3. Press the lock button (on the top-right or the right up side of your device depending on whether you have an iphone 6x or earlier). At that time, you should stop hearing your ringtone of choice and VoiceOver is supposed to come through your headphones in isolation.
4. Try to flick through the calling screen.
Expected behavior: VoiceOver would read the different options on that screen as you flick through them and would allow you to select or perform the two-finger tap gesture to answer the call.
Instead, VoiceOver seems to freeze to the point that it does not provide speech at all and so do the device (to this extent it is necessary to have visual confirmation), and it is not possible to perform the two-finger double-tap gesture to answer the call. The only way calls can apparently be answered, is by pressing the button on the remote if you have Apple headphones or others that come packaged with this feature, but under no circumstance a VoiceOver user will be able to answer the call once they have pressed the lock button on the device.
It happens when I use Bluetooth to answer the call or attempt to make them. When I finished a call, Bluetooth disconnect and voiceover goes to speaker. Hope iOS 9.1 fix this