The yellow wire on old party line service also served a secondary purpose in some cases. As well as providing the bell ground for split ringing, on some exchanges the equipment would test the resistance to ground before completing a toll call in order to identify the calling party. The ringer coils on each phone were wired slightly differently for each party to allow this to work.
Here in the U.K. we had ground-start on our two-way party lines. Because all calls (local or otherwise) were metered, the exchange needed to determine which was the originating party in all cases, and provided two separate line circuits.
A simple relay set added to party lines then connected the line to one or the other normal line circuits depending upon which side of the line was grounded to start the call. Party line phones actually had a button on top which you had to press to get dialtone.
By the way, I was looking through BellSouth's website for something the other day, and it seems that somewhere within its territory there are still some 8-way party lines in use!
