Listen to the entire telemarketer's spiel, making a note of the SECOND point.
When he asks you if you have any questions, tell him that you didn't understand anything past the second point.
He'll start there, and explain the entire thing again. Make a note of the THIRD point.
When he asks you if you have any questions, tell him you didn't understand anything past the third point.
He'll start there, and explain the entire thing again. Make a note of the FOURTH point, etc.
The trick is to keep the telemarketer on the line for as long as possible (since they get paid by how many items they sell or how many calls they make in an hour) -- and then, at the very end, to say, "Well, thanks anyway -- but I'm not interested!"
Trust me: they'll never call again.