One of the major limitations of Google Voice since its inception is that you can't call 911 or other emergency service numbers. Most VoIP services share this problem, since it's usually impossible for responders to obtain your exact location. Google is working on addressing this, but not for regular users.
A support page for Voice has been updated with new information about 911 calling. Businesses and schools in the United States or Canada can now call emergency services using Voice, as long as there is an address assigned to the number.
"If you call an emergency number while in your home country, Google Voice sends your service address to emergency services. If your organization assigns you a Google Voice number, the administrator adds your service address to your account. If you choose your own phone number, you add your service address when you sign up for Google Voice."
This is mainly for organizations that use landline phones powered by Google Voice. The company even recommends that administrators put a label on Voice-powered phones that reads, "Emergency service calls may be limited or unavailable when using Google Voice, including during a power outage or loss of broadband connection."
- Google Support