So this is what was happening... I have a "desirable" prefix and had been getting tons of calls from numbers with the same prefix - was wondering why the heck they would be calling me.
http://www.npr.org/2017/07/31/540515367/familiar-looking-num...
One thing that this doesn't mention, but which I heard somewhere, is don't call that number back and ask "Why did you call me?" In case it isn't obvious, the caller IDs are spoofed using random numbers, and the person who has the number listed doesn't know their number is being used this way.
One amusing anecdote from the NPR piece is the person who received a call with a caller ID of ... their own phone number!