I have had many Samsung phones. I didn't know there was a button to refuse calls except the red hangup button. The buttons on the side of mine just shut the ringer off you can still answer it.
Edit: I was curious so I looked up a manual for your phone and the only buttons I could see on the side are the volume/navigation keys and here is a quote from the manual.
"3. Volume keys/Menu Navigation: Allow you to adjust the ringer volume
in standby mode,
mute the ringer when receiving a call and adjust the
voice volume during a call. Can also be used to move to the top or
bottom of a menu screen."
If yours is dropping calls when you hit the volume buttons it is defective.
http://downloadcenter.samsung.com/co...lish_Final.pdf