A telephone instrument/device used for calling (traditionally a landline set; can also refer to phones in general depending on context).
古い電話機が倉庫から見つかった。
An old telephone set was found in the storage room.
部屋に電話機を置くスペースがないので携帯だけで済ませている。
There is no space to put a telephone in the room, so I just use my mobile phone.
スマートフォンの普及で伝統的な電話機は減っている。
With the spread of smartphones, traditional telephone sets are decreasing.