Party line

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

The party line is also a term used in politics describe partiasanship within political parties.