An update: Whenever they did leave a message, it was obvious that the message had started running as soon as my voice mail greeting started. The remaining pieces of messages varied from "if you want your listing", to "if you want to be removed from the list".
Obviously, they wanted someone to press a button, and I didn't. Ultimately, they gave up. They called my business phone eight times between June 24 and July 26, and then stopped calling.
... that is to say, they stopped calling from that number. I haven't checked my notes to see whether they started up from another number.