Party line: In telephone systems, an arrangement in which two or more user end instruments, usually telephones, are connected to the same loop.
Note: If selective ringing is not used, individual users may be alerted by different ringing signals, such as a different number of rings or a different combination of long and short rings. Party lines remain primarily in rural areas where loops are long. Privacy is limited and congestion often occurs. Synonym multiparty line.
Source: from Federal Standard 1037C
In politics, a party line is describes a political party's canon policy, and specific partisanship. The common phrase toe the party line describes a person who speaks in a manner that conforms to a political party's party line.