Sounds like you have a Hero and not an Evo.
The Evo has no trackball, and the sensor is to the right of the HTC logo, not the near the Sprint logo.
I assume you're running 2.1. I can't seem to find a setting to change the screen blanking option for when making calls.
Might try the Hero forum and see if anyone has any issues.
I have the EVO and my screen goes blank and I usually am using a bluetooth, and I was able to pass my hand over the front of the screen and it would come back on. BUT FOR SOME STRANGE REASON ????? It wont work anymore. I make a call, and when I am done, I have to press the button on top of the phone in order for the screen to come back on so I can end the call. My timeout is set for 2 minutes. ANY SUGGESTIONS/HELP ??
Thanks to all who posted I was able to diagnose the problem I was having with my Evo. I was having a similar problem and started searching the forums. I would make a call and the screen would immediately go blank. I wouldn't be able to get the screen back until the person on the other end hung up. It caused quite a few problems. After reading some of these post, I discovered that my new case covered up the sensor that turned the screen blank when the phone was up to your ear. I took the phone into the sprint store while it still had the case on and they could not find the problem. It would have prevented a lot of aggravation if the folks at the store new the products they sold.