I ran into a MAJOR issue with VOIP/Vonage just last night. Something else to consider that is just as important as 911.
Poison Control!!
Vonage will not "allow/process" local toll free calls. Poison Control, even though are you dailing a "national toll free number" routes you to a local office. Call won't go through. Called 911 and asked them to connect, they couldn't (not sure if this was incompetence or inability to do so (I live in Mississippi so it could be either one)). They did give us the direct dial number for them though. Still couldn't get a connection. I believe that it was because the local direct-dial is associated with the national/local toll free number.
After returning from the ER, call Vonage and they are able to dial the direct number that we gave (of course they aren't local to MS so that could be the reason). They are going to look further into the issue anyway. Hopefully our misfortune will enable us to help someone else out down the road by getting teh problem addressed/resolved quickly.