Android lets me block private numbers, but some might be genuine callers trying to get through without realising they are calling from a private number.
How can I set Android (or an app) to give a recorded voice message to private numbers?
The recorded message file can be made using Google text to speech. It would be:
(Male voice) Sorry, the number you have called does not accept calls from private numbers. (Female Voice) Please redial without withholding your number.