A great hint. I will try it on the next telemarketer who calls and then does not respond to my "Hello" because they are too busy answering the other call they made at the same time.
They called me a few times and every time they did, I called them back several times and left many messages on their voice mail. I threatened each time to keep calling them back to fill up their voice mail if they didn't stop calling me. They stopped calling me after about 12 hours. This was the second time this approach worked for me.