Party Line Service:
Up to 20 people could be connected to 1 pair of wires.
Each party on that single line was identified by a letter, and so to call that party, the exchange would be called and the number asked for would be Antelope 1 S for example. The exchange rang a distinct ring down the Antelope 1 line, signalling the party's corresponding letter in Morse code. This distinctive ring would alert all parties on the line who the call was for. Three short rings signified the call was for the party with the S letter and so on.
Telephones - Engineering and Technology History Wiki
Party line (telephony) - Wikipedia
Telecommunications Virtual Museum
last updated 2 Feb 2019