These are nice when they happen in some ways. You could be anywhere and get the call which can be very useful for a job seeker. Unlike the usual in person interview you don't even have to dress up like you normally would.
However, just because it seems like it's less formal doesn't mean that it is by any means. A lot is riding on that call.
Your future at that company for one thing.
Just like a personal interview you need to have answers prepared and ready. What I found helped me out with phone interviews was to make sure I had the same posture as I would in a personal interview. Also, I always reminded myself that it was an interview. Regardless of on the phone or in person it still comes down to the fact that it is an interview.
Many companies use phone interviews for various reasons. One of them of course is for scheduling, another is because candidates have a greater tendency to disqualify themselves. So, be on your toes when you get the call as it can happen at any time.