In the United States, when do phones display the city for caller identification, and when do they just display the state?

I have a United States phone number. Sometimes when I call people in the United States, my city and state shows up. Other times, just the state shows up. I'm wondering what sorts of things affect this. The big question is "What causes this?".

I'm sure there are loads of things, so let's also break it up a bit. Is this likely to have to do with the caller ID service, one of the telephone providers, or the software running on the phone? I suspect it has to do with more than one of these, but I would like to rule things out if I can.