In cities, if you had a party line (2 or 4 party) you had a listed
> number the same as anyone else. If someone called you, they dialed
> the listed number normally and only your phone rang. The Bell System
> used a special wiring technique to isolated the ringers of up to four
> separate parties so only the desired party would ring. (I believe it
> was a combination of bias and grounding). The independent companies
> used different ringer frequencies to isolate ringers.
Bell used diodes and grounded ringing to get four party full selective ringing. This gave plus and minus tip parties and plus and minus ring parties. REA standard companies used one of a series of frequency selective ringing schemes to put up to five parties on tip and five more on ring. I have no idea how common such large agglomerations were.
Note -- the above is for party lines of up to 4 customers. I believe
> anything beyond 4 had to be on a manual system and had to use special
> short-long ringing codes. Everybody heard the other phones ringing
> and had to listen if it was their code.
Frequency selective could go beyond four, as noted. I don't recall either scheme being able to effect ANI beyond two party, so outgoing calls had to use operator number identification, which led to the jocularly named "no test trunk". The operator would dial the number given to see if the result bridged onto the correct line.