Life's too bloody short. I have caller display. If I get a call from someone not in my contacts, I just hang up immediately if:

*no-one speaks to me for several seconds

*someone sez "Am I speaking to Mr Shaw?..."

*it's an automated message.

Otherwise I might listen for a few seconds in case it's the Tesco delivery man who can't find my remote house. Don't wanna miss out on my 25% off six bottles...

Tommy Cooper said that someone rang him up and said "Who's speaking, please?" Tommy said to him "You are." :-)