Read this and answer the question. If the question is unanswerable, say "unanswerable".

A residential elevator is often permitted to be of lower cost and complexity than full commercial elevators. They may have unique design characteristics suited for home furnishings, such as hinged wooden shaft-access doors rather than the typical metal sliding doors of commercial elevators. Construction may be less robust than in commercial designs with shorter maintenance periods, but safety systems such as locks on shaft access doors, fall arrestors, and emergency phones must still be present in the event of malfunction.

What safety mechanisms are still required, despite lower design costs?
locks on shaft access doors, fall arrestors, and emergency phones