One question I was asked, and subsequently ask interviewees, is"Describe how you would make this phone into an application". Have them describe the classes, their properties, methods, interfaces, etc. Then question them on why they chose to implement them in that specific way. It gives you a good idea if they understand how to code, and gives you some insight into how they approach and solve problems.
Also, if you offer a suggestion of how they could have implemented it a different way, it may show you whether they are open to new ideas, criticism, or if they are a team player or not.
