The phrase “party line” has three distinct meanings in American English. The first is an element of Communist doctrine. Once the vanguard of the Party had determined the truth of an issue by process of dialectical materialism, it became the “party line,” and all members of the Party had to believe it. Second, and completely separate, a “party line” was a telephone technology, common in rural areas during the early-to-mid 20th century, where a number of households shared a single line, which only one household could use at a time and on which all the households could eavesdrop. Finally, in the late 20th century, a “party line” was a phone service where users would pay X dollars a minute to engage in group chat, often of an erotic nature.
So if you had a shared phone line dedicated to erotic discussion of Communist doctrine, it’d be a party line party line party line.