The Sentry 2.0 is fantastic. It just started getting sold on Amazon a few weeks ago. $50 (don't get the previous one for $40).
We went through our cell phone contacts and Ooma call logs and made a list of legit inbound numbers. I entered those numbers into the whitelist (tedious, but not horrific).
If a legit someone that's not on the whitelist calls, they hear a message to press zero. If they do that, the Sentry box "rings" (not your phone, so your answering machine can't take a message).
Every inbound call is logged and tagged as a whitelist number, a blacklist number or not tagged. For the non-tagged, it's simple to add the number to either the whitelist or blacklist.
I have it connected "serially", so my phone is completely silent unless the caller is on the whitelist, and then the phone rings normally, the answering machine works normally.
When the phone rings now, I run over and pick it up because I know it's someone I know.