For this I was totally unprepared -- ask what types of interviews you will have and with whom so you might prepare better. The goal of this call is to see how you react to being uncomfortable so you shouldn't feel like a failure if you do -- how you continue to treat the customer is more important than your knowledge. (Their training if you're hired is very extensive.) Focus on taking care of them and satisfying their needs. You will be given opportunities to personalize your conversation (get a first name, reference a random anecdote later in the call). Also use this as a learning experience if you haven't done non scripted phone calls before!