iPhone goes to sleep while calling

'Standard' troubleshooting steps include a reset (press and hold the home and sleep/wake buttons until you see the Apple logo, ignore the slide to power off prompt) or a Restore via iTunes (from backup, so no data are lost). Else, I'd recommend a call to AppleCare or a visit to an Apple Store.

Test your proximity sensor by playing a voicemail and placing your finger over the area to the left of the earpiece. The screen should turn off and then back on when you remove your finger.

You might also try resetting your iPhone 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.

I am sure glad I ran across this entry to this forum. Just last week I upgraded to the 3GS from the 2G and began experiencing the same issues. Went to the Apple Store and they reset the original software to no avail the same symptoms. Not thinking, I had transferred the Speck full black rubber case from the 2G to the 3GS. In the mean time I had purchased a new 3GS Speck full rubber case for my wife's new phone and compared the two... Sure enough the 3GS version of the Speck Case had the entire earpiece open where the 2G version was blocked all for about a triangle of an opening. So I cut the earpiece area open on the old 2G version and no symptoms or sleep.

Thanks to all'

Try removing your case, it might be blocking the proximity sensor that turns the screen off when placing the phone near your ear.

Do you have a case on the phone? If so, see if it does that with the case removed. Cases not designed specifically for the iPhone 3G/3GS may block the proximity sensors.