How should I prepare?

I have been invited to have a phone interview with a city firm – How should I prepare?

I have received an email saying that I have been shortlisted and that I will have a phone interview. I am a bit worried as it is my first one, and a couple of my friends haven’t got through phone interviews in the past. What sort of preparation should I be doing, apart from researching the firm? I have heard a lot of firms ask ’off-the-cuff’ questions – does anyone have any examples?