I released a fun new feature for VOIP Recorder today: filters based on anonymous calls. Just set the calling number to be "anonymous" and you can block anonymous calls without ever ringing the phone. They will go to voice mail instead. You can of course use the filter to toggle the default record/do-not-record status as well.
Filters have always worked on any Caller-ID based name or number, and now they essentially work on the absence of a number as well.
Anonymous call blocking is in revision N of the VOIP Recorder preview. VR makes more out of your Vonage&trade service. Check it out at http://www.penbaynetworks.com/