I also got a message today about a phone interview for a UC. It feels great to hear something back so soon! What a relief!
Don't have anything very helpful to add... I just googled for "grad admission interviews" to review some basic questions. Maybe search for general interview questions, too. I also wrote down some questions of my own.
I visited some schools already and I have to say, it can get a little repetitive. The more specific topics and questions you can generate, the better.