It doesn’t answer your question directly, but I gave up on blacklisting over a year ago and now white list. All unknown inbound CIDs get challenged for DTMF input and get whitelisted if they pass. All outbound numbers are also white listed. In 18 months, something like 1-3 calls managed to get through.
There is one huge drawback with whitelisting that I can’t solve, and that is you must anticipate any desirable robocaller CIDs. Things like automated credit card fraud detection, utility outage notification, etc. I have decided the good outweighs the bad.
Thread: Of Robocalls and Whitelists