FYI, when you use a cell phone to dial 911, it will use any network available. I'm not trying to defend Verizon (I have crap ATT myself) but it is not solely their fault if you dialed 911 and got nothing. It just means that no networks from any company were available.
Now, if you had just dialed the number for, say, the police station, or discrete medical services, and had no Verizon signal, it won't jump to another carrier. 911 only.