This number keeps calling but they don't leave a message.

SAME HERE!

Answered call and got robotic voice asking, "If you are X, please press 1. If this is the home of X and this person is not available, please press 2." Since I am not X, I pressed 2 and the robotic voice said that they will call back later.  Another site reports that this is for Harris & Harris, a bill collector.  Caller ID just states "US Cellular"

answered and caller had already hung up

Same here. I answered and they hung up.