Did a 30 min interview which was super easy; they want to make sure you can talk over the phone without tripping over every word. I wouldn't worry about preparing for it. The important part is the skills assessments, which are a combination of quick mental math, logic, and programming logic. Definitely drink your coffee before doing these, because speed is a large aspect of the tests. If you make it through that, you get invited on-site for an interview. This was the most fun interview trip I've ever taken ; Epic takes care of everything, you get to explore downtown Madison the night before, and the interview itself isn't too stressful. For the problem solver position, you have to take a programming assessment which requires extensive ability to solve (aka I had no idea), but this test doesn't determine if you get an offer. Got an offer exactly 2 weeks after my on-site interview.