Remember, some of the above questions you should already know. Websites and agentquery.com give a lot of info, as does PW. So before the phone call, write down what you do know already so you can seem prepared and tell the agent, "I do know this about you and/or the agency. Can you expand on this?"
Questions that you wouldn't get from their website or other places:
*Would it be okay to contact a client or two of yours?
*What is the best way to communicate with you? Email or phone? (If this is for representation, not just an interview.)
*How many of your clients' books have you sold in the past year?
Good luck!