Keeps calling and leaves a voicemail with no sound or other information. I am on the National Do Not Call List, but that doesn't seem to matter to these dirtbags.
For same reason all pests do. They're either looking to cheat you out of money, personal and financial information, ransack your computer, or find out if your number has an answer. That pretty much explains why. No kind of answer, none of the above is accomplished.