Well, a call can get through once . . . then you can block it. I suspect one has to let a first call get through with any call block feature.
Google logs every call it handles, just like Vontage, and one of the options for each call is to block that number. That call will not get through again nor will it ring your phone.
Here is a screen shot of the call blocking service.
Google plays the "Not in Service" message ("We're sorry. The number you have dialed . . .") - the call will not be forwarded to your phone, thus, your phone will not ring.
Note that you can access your Google Voice message on Google and the phone Google Voice forwards your calls to and Google sends you an e-mail with the audio file and text (if it can transcribe the voice message).