@Abukana
As 9000 wrote - your proximity sensor seems to not be functioning. It is located top right corner of the device - near the front facing camera. It is sensor that tells the device that it is very close to some surface(e.g. your face or pocket walls) so it keeps screen locked.
Do as 9000 suggested - call your device and block proximity sensor by placing your finger on top right corner - the screen MUST remain black until you remove your finger. If it is showing incoming call screen with your finger ON top right corner - your device is malfunctioning, so you better bring it to repair center.