The same telemarketing company keeps calling about sustainable energy, and asking me to make an appointment. They've called four times and I've asked to be taken off the list each time. The first time, they told me there was no list, then they told me they can't take me off the list. Twice, they've engaged in an argument and then hung up. The last time, they said I had to book an appointment with them for their services so that I can be taken off their list.
If I don't recognize a number, I don't pick up. Caller left no message. Losers.
Like I tell my kids (who now have cell phones), do NOT pick up the phone unless you can see who it is (Mom, Dad, etc.). If it's important, they'll leave a message. ...besides, most of their friends just text anyway.