First, make sure you know who does the hiring at the company. Second, try calling them either at lunchtime, or just before or after normal business hours — you will have a much better chance of talking directly to the person you are seeking rather than a PA or receptionist. Thirdly, persevere — if your contact initially says there are no positions free, ask her/him if you could come in to talk about future positions; if s/he then says there is no likelihood of a job in the near-future, ask if you can meet to pick their brains about the industry.
REMEMBER THE COMMON THREADS
While you should, of course, prepare answers to questions that are specific to the job for which you are applying, it is also worth bearing in mind that most interviewers are seeking reassurance on a handful of points. Can you be part of team; be flexible; communicate well; and resolve any conflicts that might arise? Think about how you will show these qualities and you will be well on your way to getting the job.