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
I keep getting calls from 2012986979 at odd hours. When I pick up nobody speaks. When I call back nobody picks up either. I understand from the forums, the call is from Vonage. Please take necessary action against them.
It's really really irritating to get calls from this number. Nobody talks but still calling. Why Vonage is annoying sooo much...should change the service.