I often get “Private Caller” on the caller ID, and when I answer it usually turns out to be someone selling something or trying to send me a junk fax. After a quick Web search I figured out how to block these: Pick up your phone and dial *77 (1177 also works). If successful (i.e., if your phone company supports this—most USA companies do) a person trying to call you with a blocked caller ID will instead get a polite message that you do not accept blocked calls, along with instructions on how to temporarily unblock their caller ID if they really want to talk to you.
If you one day decide that you want to receive blocked calls after all, you can reverse the process with *87 (or 1187).
Everyone who reads their phone company documentation will probably already know this, but since I don’t I didn’t.