If you're on CDMA then the incall curtain will break certain functions of the phone. There's no way to avoid that because the curtain was taken from a GSM ROM. I'd recommend against it because of the losing dialing functionality.
This program is a great alternative. The screen turns off during a call but pull the phone from your ear and tilt it back and the screen turns on. Put it back to your ear and the screen turns off again. It works great for me.