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 got a call from this no 2012986979 - they know the name of the account holder and offered for a free extension. I simply said we are not interested. I can hear kids voice in the back and the guy tried to speak in an western accent but i believe he is not.
Received a call from this number yesterday around 6 PM. didn't pick up & they left no voicemail. Caller ID said it was Vonage. I'm not a Vonage customer, and this will definitely keep me from ever being one !!!