Just remember that they cannot see how you present yourself during a phone interview. It's all about first impressions. Be confident in your responses and be able to have an easy conversation. You can expect the typical questions about team work, how you're a team player, describe your strengths and weaknesses, etc.
Generally, if they want to interview you, they already want you based on your paperwork. At least that is how it was in the recent past. For the most part the interview was really to sell you on the assignment. Managers got more particular during the downturn, but it was still basically about your experience and skills which your documentation showed them prior to the interview.
From the perspective of you interviewing them, I wouldn't max out the questions. Most answers probably will not tell you what it is really like to work there - that can only be ascertained for sure by actual feet on the ground. After a couple of assignments, you will discover the questions that are important to you, for example call offs, scheduling, and floating.
In other words, don't sweat it, self promotion is not that important for a short contract - the grilling is nothing like that of interviewing for permanent staff. If you have never traveled before, or even worked at more than one hospital, don't overdo the confidence. Tell the manager that and ask about orientation and perhaps even their experience with new travelers - fishing for information about how traveler friendly they are. If she/he mentions that they've terminated travelers or have problems, ask what the issues were.
Be honest. They will almost always ask you what your weaknesses are. Don't be afraid to tell them. I always tell them that I'm uncomfortable with peds. They're small and they make me nervous! lol.. and I can get a little mouthy with the drunks. I've never not been offered a travel assignment because of this. I've been told several times that my honesty is appreciated. Because you can lie and not tell them the truth but within a few weeks of working with you they'll see the truth anyway. The manager will usually ask you how comfortable/confident you are with certain skills or certain types of patients. The biggest thing I can tell you is you just have to roll with it. You're not going to know where certain supplies are or where things are kept. But patients are patients, doesn't matter what hospital you're at. You know how to care for people, you know what needs to done. Figure out how to admit/discharge. You can learn the rest as you go.
I've had different types of interviews, from the generic what are your strengths & weaknesses, to clinical questions where I was drilled for 45 minutes on critical care patients, sometimes they look @ my resume & must like me because they'll just tell me about the unit & say I'm hired. It really varies. I am not a good phone interviewer and get really nervous actually, but I haven't had anyone decline me from an interview. Phone interviews are definitely better than in person interviews!
Mention that you are flexible and you are a team player. Say you have a positive attitude & work well with others. Tell them you ask questions if you are unsure of things, like you'd ask the charge nurse or another nurse for their opinion. I'd imagine a manager wouldn't really want someone who wasn't flexible with their schedule too. I usually just tell them I'll work whenever you need me. If you do have a vacation or something planned, make sure you bring it up during that interview.
Also not sure what area you work in, but most managers have asked me some clinical questions. Think of a difficult patient you've had and how you've handled that. If I can't think of one I just think of my last week of work and what happened and I'll talk about that. I remember my last interview she asked me about sepsis, like what orders I'd see, etc.. If you're ICU I'll give you more examples, let me know. If you don't know something it's ok. I've not known answers to questions and have still been hired.
Write down questions you have for the manager. They'll usually go over it. I usually ask the manager about orientation - how many days of hospital orientation & how many on the floor (hopefully 2 days on the unit), and where will I float, do they have any other travelers working, why do they need travelers, etc..