Answering machine functionality with 機能

Kanji Explanation: Ability and capability with 能

A murder suspect explains how when he called the victim, the phone kept ringing until the answering machine picked up.

目暮(めぐれ):: 「奥(おく)さん！ここの留守番(るすばん)電話(でんわ)の機能(きのう)は？」; “Ma'am! How does this answering machine function?”; Literal: “Ma'am! What is the function of this answering machine?”
奥(おく)さん:: 「は、はい…　十数回(じゅうすうかい)コールしても出(で)ない時(とき)は、自動的(じどうてき)に留守番(るすばん)電話(でんわ)に切(き)り替(か)わるようになってます…」; “Well... When there's no answer after ten rings, it automatically switches to the answering machine...”

機能(きのう) refers to the mechanical function of the answering machine.

Key Points

1機能(きのう) here means function or operating behavior

Megure is asking how the answering machine system works.
In this scene 機能(きのう) is about mechanism and operation, not usefulness in the abstract.

機能 (Function): 機能(きのう) means the function or role of an object, system, or one of its parts.
数回 (Several times): 数回(すうかい) means several times or a few times. It gives an approximate number of occurrences rather than an exact count.
出る (To go out): 出(で)る means to go out or leave. It is also used when something appears or is produced.
時 (Time): 時(とき) means time, a moment, or an occasion.
自動的 (Automatically): 自動的(じどうてき) means automatic or happening by itself through a system or mechanism.