The iPhone smartphone by Apple can do so many things that the fact that it can also make and receive calls almost seems like an afterthought. By default the iPhone is set to "lock" the screen after a minute of non-usage to conserve battery life and prevent accidental triggering of the touch screen icons. However when a call comes in you have to "unlock" the screen then tap another icon to answer. By disabling the auto screen lock you can have "one-touch" answering when needed.
Tap the "Settings" icon on the "Home" screen. Select "General" from the list of options. Locate "Auto lock" and tap it.
Select "Never" from the list of options for screen lock times. This will keep the screen unlocked at all times unless you manually lock it.
Press the "Answer" icon when an incoming call notification appears on screen. You can also connect Apple iPhone earbuds to the iPhone headphone jack. When a call comes in, press the button on the right earbud wire to answer the call.