Have you tried making one? It's pretty simple. Just make a new call treatment and select "From Anonymous/Private" on the right-hand side under Additional Options.
If you are forwarding from GV I have found the best place to route the unwanted calls to is Calling Card. Ideally, you would route to Error Message: Number disconnected but since that uses early media it won't work with GV. The caller will just hear ringback tone until GV voicemail kicks in.
When routed to Calling Card the call is answered so GV voicemail never kicks in and there is nothing the caller can do but press random numbers that Callcentric will reject and ultimately hang up the line.
But if you want Private/Anonymous callers to be able to leave you a voicemail then choose Error or Busy Tone......doesn't matter which you choose since those are all early media. In either case, your OBi connected phone will never ring.