noun
A telephone line shared by multiple subscribers, common in early telephone systems. Now mostly historical or rural.
昔は共同電話が多く、隣の家の会話が聞こえることもあった。
In the past, party lines were common, and you could sometimes hear the neighbors' conversations.
共同電話 is a specific type of telephone line, while 電話 refers to telephones or phone calls in general.
Compound of 共同 (shared, joint) and 電話 (telephone).