Quick Q what the fuck do you call this thing

When I was a kid my parents had a landline but we actually had two phones. One in the kitchen and one in the office. the thing about it was that, like older landlines, if someone picked up one of the phones while someone else was having a conversation, there would be a quiet click and they could hear and or engage in the conversation. I found this particularly delightful and liked to get on the office phone and breath into it until I was noticed, then I would hang up and go occupy myself so thoroughly that it seemed impossible to fake.