IPhone 3GS incoming call screen?

According to tmobile, (http://support.t-mobile.com/docs/DOC-5363#jive_content_id_Answer_calls)
When I receive an incoming call and my screen is AWAKE, then I can either hit Answer or Decline.
But when I receive an incoming call and my screen is ASLEEP, I can either slide to Answer, or just ignroe the call.

Is there anyway to make it so I can just decline a call when the screen is asleep or do i HAVE to just wait until it goes to my voicemail?

If you're running at least 6.0.1 when you get a call and the phone screen is "asleep", on the side you can slide up a phone looking icon and hit decline.