very few of the modem cards will work with the BT Caller Display...

For a more detailed discussion see 'Caller Display and Call Return' by
William Dangerfield, Simon Garrett and Melv Bond in British
Telecommuncations Engineering; Volume 12 part 3 (October 1993).  BABT have
issued a specification, BABT/SITS/94/53 draft. A copy can be obtained from
BABT on (01932) 222289.

The system described here is that developed by BT for use on the UK PSTN. It
is based on the Bellcore 'CLASS' standard. This has the benefit of allowing
CPE manuafacturers to base their UK models on those developed for the North
American Market.

The tones used are the same as the Bell 202 standard.  The tones are sent
between the bursts of ring voltage, which requires the modem to connect to
the phone line between those ring bursts.

When a call is made to a customer with Caller Display the distant exchange
requests the number of the caller originating the call from the exchange at
the other end of the C7 link.

If the call is not routed totally over C7 links (e.g. the caller is on an
old analogue exchange), or the caller is on an interconnected network for
which no agreement for the exchange of additional call information is in
force, the number will not be complete. In this case customer with Caller
Display will get a 'Number Incomplete Message'

If on the other hand the caller has deliberately withheld the number, by use
of the 141 prefix the Caller Display Customer will get a 'Number Withheld'
Message.

If the number is complete, and not withheld by the caller the number is
routed on to the Caller Display customer over the local access network. For
this purpose a V.23 sender has to be installed at every exchange
concentrator.

When a line is about to receive a call the polarity of the line is reversed
prior to the ringing current being applied. If the customer has Caller
Display additional messages are interspersed between the polarity reversal
and the application of the ringing current.

First a tone alert signal is sent and then an alternating series of '0's and
'1's lasting 250ms is sent by the V.23 sender to assist the CPE in detecting
the imminent arrival of the Caller Display message.

The Caller Display message itself contains the following information :

The number of the caller
Reason for absence of number (e.g. number withheld)
Time and Date (Can be used to auto-set CPE clocks)
Caller/Name Text (Intially only used for designating calls from payphones)
Reason for absence of caller name
Call type
The Caller Display message takes roughly 0.75 seconds to send, after which
the normal ringing current is applied to the line.

one of the best FAQ's i've found is :

http://www.ainslie.org.uk/callerid/cli_faq.htm#Q_18


-----Original Message-----
From: zantos
To: PICLIST@MITVMA.MIT.EDU
Sent: 5/27/02 4:39 AM
Subject: [OT]: Caller ID Modem for BT

I am still looking for a PCI modem that will decode British Telecom in
the
UK, Caller ID. Anyone got suggestions please? I have tried Hayes (now
Zoom)
does not.
TIA

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