I know this is a bit in the weeds, but I recently got an LG V10 on T-Mobile, and there is an option in the Calls menu that allows me to "Reject calls from all private numbers". I can also add numbers to a call reject list, as well. My previous HTC One M8 had the reject by number (but not for private numbers), as well.

My dad has a V10 on Verizon and they don't have the reject private calls option--so it would appear that some carriers are removing functionality that is otherwise available on at least some phones. Figured I'd share because I had before assumed it was a technical problem to block the private numbers, and have been happily surprised now that I don't get calls from "Unknown" anymore.