(update) I've gotten one call per day since I first reported this a couple days ago. Annoying that someone would waste their time doing this. I've gotten to the point where I just ignore it.
In the past week I've gotten at least four calls from this number; especially annoying is that I got one yesterday, and another today; today my phone rang once, then a few seconds later it rang three times, as if they had trouble getting through. I answered yesterday and heard somebody hanging up an old-style handset, as if someone had to actually put it down into an old desk phone cradle. From now on I intend to just let it ring; I have a caller ID so will be warned. I tried blocking the number yesterday via ATT call screen and apparently it's an invalid number.
(update) I've gotten one call per day since I first reported this a couple days ago. Annoying that someone would waste their time doing this. I've gotten to the point where I just ignore it.