According to this article and video, new Verizon phones now emit a fairly loud audible alarm when you dial 911! There have been several cases (including one in the video) where people have been able to surreptitiously dial 911 during a kidnapping or carjacking and secretly give clues about where they are and who their attacker is, but don’t try it with one of these Verizon phones because the kidnapper will be immediately alerted! It’s supposedly for accessibility, but it seems like a case of the cure being worse than the disease. Forum posts report some Cingular and Sprint phones also have the alarm, and supposedly it’s a feature you can’t turn off.
I’d like to think if you are able to sneak out your phone and dial 911, you’d be able to sneak out your concealed pistol and just shoot the bastard instead. If that’s not an option when the situation arises, be careful dialing 911 doesn’t get you killed.