Be honest. What's the first thing you think of when you hear "phone interview"? Most people won't think about how nice it will be to take the interview in their pajamas. But you should think about what you'll wear.
You should dress the same for a phone interview as you would a face-to-face interview.
Yes, even for a phone interview. That may sound crazy. But you are interviewing after all. Don't you want to be at your best.
You'll come across more confident and more professional. Plus, what if they want to Skype with you?