dr dre microphone not working well with iphone 4?

Hello, I just recently got my Iphone4, When I used it with my Dr Dre
headphone, it works fine; but it seems when I try to call some using
the dr dre headphone(using the cable with the mic) the people say they
cant hear me properly. :S why is this?

I tried using it on my Iphone 3g S and everything seems to be working
fine?

PS. My Iphone 4 is still in its plastic case. brand new fresh