Here's the explanation of the 911 Feature for Vonage. It makes sense, I guess. Basically, since the phone service is web-based rather than a landline, if you call 911, they have no idea where you are. You have to sign up for the service and register your physical address, and if you move, taking the service with you, you have to change the registration address as well.

Certainly worth checking into if you have a web-based broadband phone service, and being sure that it's activated.