How to Reject a Call on an iPhone
Rejecting a call on the iPhone is a simple process that can be completed in two different ways. When you reject a call, the call is sent directly to voice mail, where the caller can choose to leave a message or not. To check the messages of your rejected calls, simply touch the phone icon and then the voice mail icon. Giving callers special ring tones will help to alert you when an unwanted call is coming in.
Press the red "Decline" button when you want to reject an incoming call. This will only work if the iPhone is unlocked. If you are playing a game, listening to the iPod or surfing the Internet and you receive an incoming call, your program will be paused and the name or number of the person calling will be displayed. Two buttons, "Accept" (green) and "Decline" (red), will be displayed.
Press the sleep/wake button twice to decline a call and send it directly to voice mail if the phone is locked. When the phone is locked and a call comes in, the screen will display the name or number of the person calling and the option to slide your finger over the bottom of the screen to answer.
Press the sleep/wake button once to simply mute the sound of the ringer. The caller will still hear a the phone ringing on her end, but you will not. The caller will be sent to voice mail, where she can leave a message.