I don't know anyone who cuts off their regular phone service without first testing the water with Vonage. If you are getting Vonage or similar coverage just to save a dollar or two, I believe you are wasting your time.
I planned to try Vonage because my phone lines were being hacked by a hacker. By using Vonage, I was told I could make calls in private. However, I have read that hackers can also hack into Vonage lines.
Those I know who are using Vonage seem to be happy, but everyone has a cell phone and other phones while they are using Vonage. I never thought about there being a problem while using Vonage that you cannot dial 911. With the government stepping into the picture, who knows what will happen to online phone service.