Flip the Ring/Silent switch to change between ring
and silent
modes.

When set to ring mode, iPhone plays all sounds. When set to silent mode, iPhone doesn’t ring or play alerts and other sound effects.
IMPORTANT: Clock alarms, audio-dependent applications such as iPod, and many games still play sounds through the built-in speaker when iPhone is set to silent mode.
By default, when you get a call, iPhone vibrates whether it’s set to ring mode, or in silent mode. If iPhone is set to ring mode, you can silence a call by pressing the Sleep/Wake button or one of the volume buttons once. Press a second time to send the call to voicemail.