In article ,
Peter Parry writes:

The phone firmware - also for "unavailable" and "withheld" (which is
why different people see different words on different phones - it
depends what the manufacturer has put in so withheld may display as
"private".

They are passed as a flag, and also in the text field of
the callerid data. (Not all callerid devices will display
the text field, as BT abandoned using it for peoples' names
very early on after a trial in one area.)

There are a number of occasions when BT proves additional
info in the text field of the callerid data, as I described
earlier. (Actually, I missed one out which I've seen,
RINGBACK, which also mirrors the ringback call type in the
callerid data.)

CLI is passed in the UK between the first a second ring
as a tone burst and comprises only numbers.

That's how the BELL system works. BT sends the data before
the first ring, preceded by a line reversal. There's also
an off-hook signalling method which is used during call
waiting, but that requires that the phone indicates it can
take the data when off-hook, or it's not sent.

--
Andrew Gabriel