If you’re not available, due to illness or vacation, for example, how will your patients be cared for?

This is a straightforward question and the answer is always part of the by-laws of any institution. If you're not new to the area, you should identify others on staff with whom you expect to have a reciprocal on-call relationship; if you're new to the area, you should state you intend to be full-time responsible until you can establish such reciprocal relationships. An extra overture would be to say you plan to serve on many committees in order to meet others in your specialty.