what caller ID is for

When will I learn not to pick up the phone if the caller ID says "private" and doesn't give a number? I had just gotten off the phone with A and thought maybe she was calling back for something. No, it was a f***ing telemarketer calling before 9am and no matter how many times I said "No" I think he is still going to send us the Times Sunday for the next 8 weeks and send us a bill. F**k. F**k F**k

grrr.