Here is what Vonage says about their 911 service:
formatting link
Vonage routes your call to the Public Service Answering Point (PSAP), which provides emergency services in your area. The appropriate PSAP is determined by the physical address you supplied when you configured 911 on your web account. Therefore, if we do not have the correct address, your call cannot be routed to the corresponding PSAP for your area. Another difference between Vonage 911 Dialing service and traditional
911 service is that the Vonage call will be routed to the PSAP's general access line, which is different from the 911 Emergency Response Center. You will need to state the nature of your emergency promptly and clearly, including your location and telephone number, as PSAP personnel will not have this information at hand. PSAP personnel can help you effectively and will take necessary steps to provide you with the appropriate assistance, such as dispatching police, an ambulance and/or a fire truck.
Behind the scenes, the call will go to your local Public Service Answering Point immediately, if you have provided your address by configuring 911 on your web account. If you have not configured 911 on your web account, you won't be able to dial 911 at all.
*Note - Customers in Rhode Island who activate dialing 911 will receive an emergency calling service similar to E-911, which utilizes the E-911 call routing system and automatically displays your address and the number you're calling from on the dispatchers terminal. This E-911 solution is only available to customers in Rhode Island who have activated dialing 911. Vonage is working aggressively to provide this solution in additional states before the end of 2005.