This is a proximity sensor problem and here's the concrete solution that worked for me:
1. On the phone keypad enter the following code: 1809#*900# XX bring up the engineering settings menu.
2. Scroll down to the entry called OTHERS and select it.
3. Scroll down this menu to the entry called PROXIMITY and select it. 4. You should see three options: Enable, Disable & Start Calibration
5. Select Start Calibration - after a few seconds it will return a new figure (mine was 3627)
6. BACK right out of the menu system, and make a test call - it should blank the screen when you hold it to your ear (or when you hold a finger near the sensor), but come back on again when you remove it.