Go into the phone settings and set the menu button to wake the phone. This will allow you to wake the phone without disconnecting.

 

I can't find how to do it either.

 

Whoops, I've been using Cyanogenmod builds and it has cyanogenmod settings in the menu and an option to wake device with menu.

You can check under the menu settings to see if there is an option to wake device. There should be something in the settings that allows you to do it with either menu button or trackball. Also, you can set the screen timeout while in call to las longer. Sorry about that.