We had numerous calls some time ago where the phone would ring, but there would be no one there if you picked up.
We learned much later that this is due to a practice by telephone "sales people" with automatic dialing equipement that dials several numbers at once - to assure that their people will always have someone to talk to.
Although it is considered "harassment" if you or I make anonymous phone calls, and may be prosecuted, these "people" have a large lobbying effort, and my regional legislators have decided that it's okay for them to do it "because it would cut into their profits if they couldn't."
I note that caller ID will do nothing to really help, since "these people" always block the ID. My phone company has pointed out that I can get "call blocking" that will not ring if the ID is blocked (for about $4 per month), but it took me about 20 minutes with the phone co rep to confirm that "call blocking" will not work unless you already have, or also add, "caller ID" for another $6 per month. Since I have two phone lines, I am told that I can't get either unless I get them for both lines.

John