You're thinking of cell phones. If I shut down service completely on the home phone there's no dial tone at all - no way to call 911. Plus, the right way to do a VOIP install is to unplug the home telephone network from the telco where it comes into the house and plug the VOIP adapter into the home phone wiring at any jack - then you can use any phone in the house just as though it were connected to the telco.
I read Vonage's FAQ on 911 calling - says you still need to be able to tell the PSAP operator location and nature of emergency - I don't trust my 4-year-old to do that if I'm unconcious.
Scott
