noun
add-on telephone
A telephone device that connects to an existing line, often used for extensions or additional handsets.
このアドオン電話は、親機に登録するだけで使えます。
This add-on telephone can be used just by registering it with the base unit.
子機 is a more general term for a cordless handset or extension unit, while アドオン電話 specifically refers to an add-on telephone device.
From English 'add-on' plus 電話 (telephone).