I recently updated my iPhone 6s from iOS 10 to iOS 11.2.
Together with the update the device started really annoying me by turning on Voice Control on headset button press. It gets turned on approximately in 30% of the cases. May be it reacts on a longer presses, but anyway I'd better turn this off.
I used to use this button to play/pause music and it worked perfectly with iOS 10. I also didn't change my habits - I press it in exactly the same way like I did it before.
Voice Control itself is turned off on my device. I used the "Turn off Voice Control on your iPhone" article from the apple documentation to ensure I did it properly.
There is also quote in the documentation:
If you're using a headset, and it unexpectedly activates Siri or Voice Control, make sure that your headset is fully inserted into the device.
But it is not my case - the headset is fully inserted. I'm also using the headset provided with the device.