Did Apple say this somewhere? The background noise reduction works best when you hold the phone to your head, not with a headset. It works because there are two microphones on the phone, one on top and one on the bottom. They both hear the same background noise, but the microphone on the bottom hears you voice a lot better. So the signal processing subtracts the top waveform from the bottom waveform--the noise cancels out and your voice is left. Headsets can't do this.