Hi Shiva
Not sure what the background to this is but based on what you say and a few friends' experiences I would say that the interview is another step in the appointment process. Someone will be offered a position, based on the outcomes of the interviews. With any luck it will be you! You clearly have a chance otherwise they would not bother with the interview, so don't worry about the GPA - they must see promise in your experience/proposal. If you are applying for a PhD, yes - be prepared to talk about your proposed topic and link it strongly to your previous experience. Start off addressing them formally (Good morning, Professor X, or Dr Y); if someone says, 'Call me John', fine, take their cue. Work out any time differences. Research the staff lists on the department website to see who is there and what their roles and research interests are, and how they fit with yours. Lastly, have some water handy by the phone. Wear a smart outfit, including shoes, so you feel work-like and formal. Somehow no-one sounds the same in pyjamas!!! Have fun, and the best of luck :)