hello ashhad,
I wrote to someone about such a problem a moment ago - it still happens. Make sure that the proximity sensor is not working: call your inbox and cover the mesh of the speaker with your finger during the call. If the screen turns off - the sensor is working. Additionally, make sure that the light sensor is working - put a working phone under a strong light bulb - if the screen brightens by itself, the sensor is working. If any of these tests fail, write - I'll tell you what to do next.
In general, you will need to test voltages and resistance check the entire tape of the talk speaker along with the sensor bank and the "Flood Illuminator" sensor.