We never answer land line or cell calls if caller ID not recognized.  Let it ring, as it wastes their time and saves ours.

Called, did not leave message.

Called tonight but left no message.

Called, no message. From Arizona.

I found this statement on another site:

"Spoofed Numbers: Caution, 480-376-8643 is a "spoofed" number, which means that whoever called you "tricked" your caller ID to display an incorrect phone number. Telemarketers do this so you cannot complain, but they failed... you can still file a complaint here to fight back against unethical telemarketers."

I guess they just keep changing numbers from one spoof to another.  You would AT&T and their ilk would be sophisticated enough to detect and prevent "spoof" calls.  Maybe in the next century.