When my husband and I owned our business we would play dumb and ask them to hold. Then wait at least 10 minutes, pick the line back up and ask "who are your holding for?" and put them back on hold. Repeat until they give up and hang up. Worked great for us. One secretary we had used to mess with anybody selling anything saying that she had a brother who sold that very same thing, insurance, copy paper, printing, etc. She was good!