It's not a feature with HTC.. The power button is used for waking up the phone. You can use an app like Spare Parts but you will need to adjust a few settings within the app. Pressing the power button will end the call which you may not want, if you just need to wake up the screen for something while on a call. You will have to set another key to wake up the phone.
Another possible way would be if you are rooted and use a custom ROM. Cyanogen Mod has the built in feature for using the power button to end calls, once again though you will need to use some kind of app or widget if you are just looking to wake up the phone while on a call.