Actually they can and do. Their automated dialers plow through all numbers. If someone answers, the call is transferred to a live person. If you ever picked up, said "hello" and listened to blank air for 2-3 seconds until someone came on, you experienced this technique.
Therefore, if I say hello and get no immediate response, I hang up immediately.
Incidentally, I have a second telephone line so that calls coming in when line 1 is busy kick over to line 2. The number for the second line is unlisted and I haven't given it to anyone. Anytime I see a call coming in on that line, I know with certainty that it's a computerized telephone dialer. Needless to say, I don't even pick up.
You are missing out on one of life's great challenges--trying to get the telemarketer to hang up on you.
Gee, I don't have many friends--will you be my friend?
Are you wearing underwear?
What kind of underwear are you wearing?
Will you sell them to me?