The iPhone is a phone right, so you probably get a lot of calls. You should probably know that pressing either of the volume controls while the phone is ringing will silence your phone, but continue ringing for the caller. Whether on vibrate or loud, the phone will become completely silent.
In addition, pressing the On/Off button at the top of the phone will send the call to voicemail. So instead of your phone ringing away, just press the top button to send them straight to voicemail.
"iPhone Tip– Manage Incoming Calls"
Reference : online-tech-tips.com