Here's one way you could do it. Place a security lock on your screen and it will automatically default to the home screen if you end a call. Mine does it all the time, but of course you'll have to unlock the screen in order to complete any other tasks on your device. Here are the steps to do so.
Setting a Lock Pattern
1. From the Home screen, press the Menu Key
2. Touch Settings > Location & security
3. Touch Set unlock pattern (under the Screen unlock pattern
section of the settings). The first time you do this, you will
see a tutorial with instructions on how to create an unlock pattern.
4. You're prompted to draw and redraw your own pattern.
*From then on, any time you wish to turn the phone on or
unlock the screen, you will need to draw your unlock pattern to
unlock the screen.
Hope this helps