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 use other apps. To return to the call screen, swipe the status bar down and touch On-going 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.