Get more information on using the iPhone Phone app in this free lesson.
When you receive a phone call, you can choose to answer or decline it. To answer an incoming call, slide the green bar. If you want to silence the ringer, press the Sleep/Wake button. To decline the call and send it directly to voicemail, press the Sleep/Wake button twice.
If you're already using your iPhone, you'll tap Answer to answer the call or Decline to decline it.
You can also tap Message to reply with a text message or Remind Me to set a reminder to call that contact back later.
Many apps can be used during an active call by pressing the Home button and selecting the desired app. When you're ready to go back to the Phone app, tap the green bar at the top of the screen. Note that some wireless providers only allow you to use this feature when your iPhone is connected to Wi-Fi, not 3G/4G.