Always review the job posting before the interview to familiarize yourself with the duties involved and preferred qualifications for the successful incumbent.
Answer this question by mentioning the largest switchboard(s) you have had to operate - including the one(s) listed on the job posting if you have such experience, to demonstrate you are indeed the perfect candidate for the job by virtue of your previous experience - and the company(ies) you were working for.
Should you not have such experience, emphasize you are a quick learner and that you have a willingness to learn.