Ok, now understand;
we need to turn the vibrate ON so when volume button pressed down, it goes into vibrate mode (or by quickly pressing the bell button on right top corner).
when the vibrate is OFF, then volume button pressed down put i into silent mode (or by quickly pressing the bell button on right top corner).
However, if vibrate is turned ON then the phone vibrates alongwitht he ring for incoming call. What I'm looking for is simultanous options for silent and vibrate. I think it's missing in Windows 7.