The mute button on IPhone4 is located close to your left thumb in normal conversation mode. Sometimes with unintentional thumb movement, the button slides to 'mute' mode easily. Thereafter, you do not get any audible rings because of the button being on mute. There is no red light or visual indication to show its mute status. How frustrating it is to miss out your incoming calls until such time you realise and reverse the slider back to normal. Is there any help available or is it just a design problem? Personally, I feel that the best location of this mute button was on the top side of the phone where there is no possible interference along with a red light indicator for mute. This would help to see & make sure the status of mute button even in dark.