What is an "Emergency Call," and can I set number for it?

On the default lock screen, whether using pattern or pin, there is a button at the bottom which says "Emergency Call". But if I try to call my home number or anything it tells me "That is not an emergency call." What ***does*** it let through? Is it only 911? Is there any way to tell it to allow a certain set of numbers from the emergency button, like to set a list of emergency numbers such as: 911, Home and Wife? This would seem like it would be very useful, so if someone finds you smooshed on the road, they know these certain people should be called.