(Typing) The technology clearly works if it gets a clean enough signal from the microphone. The microphone on the cheap headset provided with DNS is supposed to be 'noise-cancelling', a bit like ones used by helicopter pilots. It seemed to be rejecting most of the sound from my TV which was about 8 ft from the mic.
I'm sure that you can buy much better headsets for £40 or so and I will buy one when I have the money to spare.
I know that some mobile phones have noise-cancelling technology built in, but I don't know how effective it is. Pause to consult Google ... Okay, here is a demonstration:
I thought it worked really well in the music and the air conditioning tests. I don't think the wind test was a fair one because no mics work properly in windy conditions without wind covers on them, which mobiles don't have.
DNS works best when it is trained to the combination of an individual voice through a particular mic setup so I could see it working on the iPhone in the demo, if it got used to the noise-cancelled 'robo-voice'.