Your screen goes dark to avoid accidental touches. To wake it up, move it away from your face or press the Power button.
During a call, you can:
Multitask. Touch to hide the call screen and look up a phone number or use other apps. To return to the call screen, swipe the status bar down and touch Ongoing call.
Answer another call. On the Incoming call screen, if you answer the second call, the first will be put on hold. If you respond with a message or send to voicemail, you return to the first call.
Adjust the volume. Use the volume button on the side of the phone.
Use speakerphone. Touch .
Use a paired Bluetooth device. Touch to use or stop using the device.