My LCD does not turn off during the call. I have a iphone 4. What do i do?

Try removing your case, it may be interfering with the proximity sensor.

You may have a defective proximity sensor but try some basic troubleshooting first...

Try a reset by pressing the home and sleep buttons until you see the Apple logo, ignoring the slider. Takes about 5-15 secs of button holding and you won't lose any data or settings.

If still a problem restore with your backup.

If still a problem restore as new, i.e. without your backup. See how it runs with nothing synced to it.

If still a problem, it's likely a hardware issue.

I know Google Voice has a bug that doesn't turn off the LCD but otherwise on a "normal" call try the above advice.

See this... http://support.apple.com/kb/ht1766