Can you use the back power button? You can check in call settings to see if you have enabled or disabled the "Power button ends call" option. If it is unchecked, it will turn screen on. Other than what you have tried, this is the only other way I know to turn the screen back on. Screen is supposed to come back on briefly when the proximity sensor registers that you have removed the phone from your face.
It sounds like the proximity sensor is not functioning properly. This could be due to a case or sticker covering it, or a defect in the sensor. How old is the phone? Has it been wet, or dropped?
The proximity sensor will darken the screen to save battery when in a call, but if you pull it away from your face to enter #'s on a key pad, for example, the screen comes back on. It's usually located near the top of the phone.
Some cases actually fit both the VZW and International versions, they just have larger cutouts on the back for the buttons. Problem is, is the sensors are also slightly different in location between the 2 variations. Some cases are a bit too close to the sensor on the front.
Have you tried just giving it a shake, kind of like moving a mouse to wake up a desktop? That usually works for me. Not always, but usually. I appreciate everything that conserves the battery, but it's annoying, especially when dealing with an automated menu system that requires me to use the keypad.
Yes a crack-the-whip type gesture will wake my screen, so I can use the keypad while in-call.