AFAIK, party lines were when N people were physically hooked up to the same phone line. All N people could talk to each other at the same time without dialing out/having to pay.

https://en.wikipedia.org/wiki/Party_line_(telephony)

My family had a party line when I was growing up. The other parties who shared the line weren't very happy when 13-year-old me got his first 300 baud modem.