Old home landlines just had a single line for the whole house. If you picked up a phone, you could listen to and talk on whatever the conversation was.
Plenty of affairs were discovered by the wife picking up the phone to make a call while the husband was in his home office.
Party lines were especially popular in rural areas; Dozens of farmhouses would share a single party line, so farmers could just pick up the phone and chat with whoever happened to be on the phone already. It was a huge source of socialization for people who otherwise would have been almost entirely isolated. Farmhouses often have literal miles in between each house, so socialization was difficult simply due to the distance. Party lines were basically pre-internet Discord servers, and you just shared the server with all of your neighbors.
Many farm houses had two phone lines coming into the house; One private line for personal calls, and a party line for the neighbors.