There are few alternatives to block such calls. May this options in the link below will help you.
http://vontech.blogspot.com/2012/12/vonage-specific-call-block-feature.html
Vonage Customer Service. I could hear people and equipment working in the background but they didn't say anything to me when I answered the phone. Yes I have an account with them but that doesn't make it ok for them to call and not say anything.