Chris Lewis wrote:
According to HorneTD :

Sorry Art but you are the one that is wrong. When touch tone dialing
was first provided on subscriber lines polarity was how the use of TT
was limited to those who paid for it. The old 500 desk sets and similar
phones of that error had polarity specific touch pads. The polarity of
the line was reversed at the protector to prevent the use of touch tone
telephones on lines that had not paid the fee. Many residential
premises have reversed polarity on the internal wiring as a legacy of
that original practice.


Some undoubtably worked that way.

Others didn't - it depends on the switch and telco. Some switches you simply
enabled/disabled tone on a line-by-line basis.

[We make telco switches (ie: DMS) that do that ;-)]

Line polarity was (like 30 years ago) also apparently used for a simplified
"2 customer" party line scheme. Where, instead of distinctive ring, the ring
was transmitted down the line in just one polarity, and only the phone with
the diode the right way around "heard it". My grandparents had that at
a cottage, and so apparently did my inlaws for a time.

Aside from these prehistoric setups, these days it rarely matters which
way around tip/ring, red/green are. Better to have it the right way around,
but...
Since ringing current is 20 HRZ AC that would not work. What was
commonly done in early practice was to connect the ringer between the
earth and one side of the line. In that way the ringing current could
be sent on tip or ring depending on which party had been called. That
system of ground return ringing was used on multi party lines so that
only half the subscribers would have to hear each ring signal.
--
Tom Horne