I used that trick long before Barnacles suggested it.
I first thought to do that after I got my first job as an intern at an insurance agency. I used to do cold calls through our calling list (I hated doing this part of the job) and would ask people if they wanted a quote.
Sometimes new numbers would be added to the call list. Occasionally we would get a fax number, and at that point we would drop that number from the list. Other than that, we would never take the number off the list. Even if the person asked us to do so. We were instructed to simply set a reminder in 6 months to try again.
I hated that job so much. People don't call their insurance agency unless they are unhappy or want to make a claim. In either case they are probably distressed or angry.