What I did on my OBi110 was to setup another GV number and record a message (a gan one) saying that this number does not accept calls from blocked or unknown numbers. I recorded this message about 5 times in a row. I also enable "Do Not Disturb" on this GV Number.
I then setup the inbound call route on my main GV number to divert all anonymous/unknown numbers to this GV number. After awhile, I notice few to none of these calls.
SPx Service >>> Parameter Name >>> X_InboundCallRoute >>>....,{?:spx(1ABCddddddd)},........, where 1ABCddddddd is the number of the 2nd GV number. You could divert these calls to other numbers also.