This is the explanation I was given by someone who used to work in a call center, basically. We always wait at least two rings before answering, and this usually means someone else answers before we do and gets the sales pitch, while we get silence. The no one there calls are annoying and time wasting, but not as much as the ones where the salesperson is actually on the line.
I like Catherine's solution too. I used to, before the do not call list, answer my phone at home with "we do not accept telephone solicitations at this number." I think the formal language threw them off stride, because quite often they just hung up after that.