When a call comes in, slide the on screen lock slider and your talking. But, if you do not want to talk now, you can click the ‘sleep’ button on the top of the iPhone to stop the ringing. If you tap the ‘sleep’ button twice quickly, the caller will be sent right to voice mail.
You can also stop the ringing by tapping the up or down volume button on the side of your iPhone. The flip switch just above the volume buttons is the ringer silence so no ringing in the middle of your meeting… or movie.