well, when someone calls and then the ringtone rings,
when i either press volume up or volume down as the ringtone plays when someone is calling. the RINGtone stops....but its still receiving the call. o_O
i believe that the volume up/down buttons silence the ring in event you don't want to answer the call. you can still answer it if you have a change of heart before voicemail picks up. i could be wrong as i never use this feature. im on vibrate almost always.