Ralph Mowery wrote:

US standard ring voltage is 90
volts AC at 20 Hz unless you're on a party
line and the ringers will be of the type
that are filtered to ring at different
AC frequencies.

Multi-party service did NOT use "filtered ringers". I don't believe
such a thing ever existed.

On two-party service, the ringing current is sent down one "side" or the
other of the serving pair. At the station, the phone's ringer was
connected to either the ring or tip side of the pair and the other side
to ground. As mentioned earlier here, most station wiring was, for
DECADES, three conductor. The third conductor was to provide a ground
for partyline use.

A private line-wired set, "illegally" connected to a 2FR, would ring for
ALL calls because its ringer was wired ACROSS the pair instead of as I
described above.

Really OLD, multi-party installations? That was even more complicated.

A 4FR (four parties on the same cable pair): One party on the ring side
was assigned a LONG ring, the other party was assigned two, short rings.

The same held true for the two parties whose phone ringers were wired to
the TIP side of the pair.

This way, the phones would ring for only TWO parties of the four.

Then there's six and eight-party service. It's just more of the same
with either three or four parties on one side of the pair and the others
wired to the other side of the pair.

Then they got into different ring patterns such as we call today Custom
or Distinctive Ringing: [long]; [short]; [short short]; [short long
short]; [long short long]; and so on.

In other words, with eight parties (with properly wired telephones) on a
single pair, only FOUR would hear the ringing of partymates.

I missed those days by a decade or two. I didn't miss much, methinks.
--

JR