You can hit the BACK button after initiating the call and that will dismiss the call screen. You can get back to the call screen by going to your notifications and selecting it there.

The Droid should have a proximity sensor that detects when the phone is up to your ear and turns off the call screen. You can test this by putting your thumb near the speaker while in a call and looking at the call screen. If the screen does not go off you may have a defective phone.