How to lock my touch screen when I am in a call?

I have a HTC touch pda phone (in white). Whenever I am on a call, because it is a touch screen, I always accidentally pressed on the ons that is on the screen, especially my loud speaker on or the end call on. I always end up hanging up in the middle of my phone calls and it's so annoying! Is there any software that can deactivate the touch screen every time I am on a call. I know iphone does that. Can anyone help me please?! =(