iDevice Primer 105: How do I answer an incoming call, or end a call?
For the most part, you can both receive and end a call with one gesture: the two-finger double tap. tap two fingers on the screen, then tap them again quickly. When you are receiving a call, this gesture will answer; when you are in a call, this gesture will end it. When ending a call, though, please note that you must remove the phone from your ear and/or lay it flat. This tells the phone to exit the mode where the touch screen is basically turned off, to prevent your face from activating commands as you talk.
Sometimes, for reasons unknown, this gesture will not work. In that case, simply find the "answer call" button that will appear on the screen near the home button. Alternatively, bring up the phone app (which will launch when a call is answered anyway) and manage the call from there.