Your proximity sensor may not be working. If you cover the sensor when the phone is on call it should go dark, which is similar to you putting it close to your face/cheek during a call.
Open the device and check your work. It could be one of many things.
-Cable is not completely seated
-Damaged cable
-Sensor failure/defect - Good to test on another device to make sure.
- damaged pins on either board or cable
good luck