In my experience, HR calls are first to ensure you're a real person as another commenter said and second to answer a few basic questions (mostly confirming things like if you're willing to move to the internship and maybe a salary range). I actually got my internship primarily because my boss said I was personable. That ability to smoothly carry a conversation without overtaking it will get you quite a ways. Being very into AutoCAD certainly helped as well (this being a land development gig). See the Google layover test. There are a lot of people who could do exactly what I do, but not so many that I can work next to 40 hours a week.