How do I answer an incoming call on my ZTE Avid 557 when screen is black

SOURCE: Screen Blacks out on receiving calls

You need to update the software/firmware of your phone. Doing this usually solves the problem. if it doesn't then you need a new screen.

SOURCE: i cant answer an incoming call, the screen goes black

you need to recalibrate the proximity sensor. this is what stops you from pushing buttons when you are in a call ect. to do this go to main menu > phone settings > proximity calibration. from here you can recalibrate the screen which should fix the problem