There are a lot of posts here about problems with the proximity sensor. Stand in front of a mirror, or have a friend watch when you are talking. The screen should turn off when you put the phone to your face, and should stay off until you pull the phone away. If it does not, borrow another droid, and try the same test.
If yours has the problem, and another droid does not, return it to verizon, they will replace it.
My wife had the problem - it made the phone almost unusable. The prox sensor did not keep the display off, so her cheek would put the phone on mute, on speaker, or hang up. ALL the time. Was quite annoying. Verizon initially kind of gave her the runaround, but when I showed them the test above, they immediately agreed there was a problem, and she had a new phone. The new phone works perfectly.
jc