RaspISteve
Posts: 98
Joined: Wed Oct 24, 2012 9:15 pm
Location: Cheltenham, UK

Domestic Caller ID Revisited

Thu Aug 02, 2018 10:07 pm

Guys,
Just did a little search here looking for Caller ID. Seems lots of folk are having either little success, or just want it. A few have an expensive or hard to find modem which can do it. So far nothing really definitive. Being retired and at home I get a lot of calls from folk wanting something or other from me. So, at the risk of incurring the wrath of the self appointed for re-lighting an old theme all be it from the top again I though it worth sharing my experiences.

The big problem seems to be sourcing a modem that can actually do Caller ID Decoding and specifically UK-BT's version of Caller ID which is a little squirt of V.23 just before the first ringing cycle. The problem with Caller ID signaling is that it appears before the first ring cycle which means the modem has to be ON-HOOK and line therefore open to a new call. From what I've found modems are generally designed to only operate OFF-HOOK unless that is it is specifically designed to support Caller ID. Basically lots of modems but only a very few can do Caller ID, certainly none of the really old types.

In the 21st Century and with Broadband there could well be a surplus of old IBM-PC modem cards or other surplus modems that include the basic AND APPROVED circuitry to make the necessary and SAFE connection to the line. Certainly there are circuits shown on the web although few warn of the need for components with sufficient voltage tolerances to protect both sides from each other. Maybe subverting the passive front end of an old surplus modem is a way forward simply making an audio connection after the modem's line isolation transformer. Whatever the connection is it must NOT provide a DC path through itself or the exchange will detect an OFF-HOOK condition and block the line to incoming calls. It must also provide a quite high impedance AC path else there could be adverse implications for the line. A low value capacitor rated at 250VAC or higher could be a solution here.

I then found:

Code: Select all

https://github.com/aquila12/v23
This is a software modem that performs the V.23 functionality using an ALSA supported sound card. Once compiled on my laptop and using two Xterms I could pipe a file into an instance providing the V.23 Modulator to sound-OUT via a link into sound-IN to another instance providing a V.23 Demodulator and the transmitted file would appear on the second terminal.

Transferring to a Pi3B it ran but barfed on the C-Media CM108 sound interface. A bit of research and a small update and this was resolved.

I also found:

Code: Select all

227v3p7.pdf
242v2p5.pdf


These two files describe BT's Caller ID structure and protocols.

My solution here was to add the Caller ID functionality. I did this by also including a Caller ID encoder function for testing. This was simply a pre-encoded block of data that got passed into the Modulator instance. Next was a function to take the Demodulated V.23 data and decode the content to standard-Out.

What I now have is a laptop able to generate a recurring squirt of V.23 encoded BT Caller ID and a Pi with a USB sound dongle decoding this and piping the content to a file. If this is to go anywhere I will need a means of safe and approved connection to my telephone line.

Turns out that one of the threads I found was someone wanting Caller ID as a school project. The route I've taken thus far would satisfy this one while also allowing the demonstration of simple V.23 Modulation and Demodulation and without risks of messing with a phone line.

Before anyone asks... I don't currently have any means of sharing what I've done but if an old guy can do it given the clues above it should be do-able for others.
Share and Enjoy.

gkaiseril
Posts: 654
Joined: Mon Aug 08, 2016 9:27 pm
Location: Chicago, IL

Re: Domestic Caller ID Revisited

Fri Aug 03, 2018 1:42 am

You will need some circuitry to tap onto the POTS line and decode the FSK signal into the 16 possible characters. In the beginning this was done with a modem that connected to the POTS line and then fed the signal to the computer via a an RS-232 cable.
f u cn rd ths, u cn gt a gd jb n cmptr prgrmmng.

User avatar
DougieLawson
Posts: 36322
Joined: Sun Jun 16, 2013 11:19 pm
Location: Basingstoke, UK
Contact: Website Twitter

Re: Domestic Caller ID Revisited

Fri Aug 03, 2018 7:01 am

In the UK you can't connect any equipment to the POTS without having BABT approval.
Note: Having anything humorous in your signature is completely banned on this forum. Wear a tin-foil hat and you'll get a ban.

Any DMs sent on Twitter will be answered next month.

This is a doctor free zone.

hippy
Posts: 6096
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: Domestic Caller ID Revisited

Fri Aug 03, 2018 10:28 am

DougieLawson wrote:
Fri Aug 03, 2018 7:01 am
In the UK you can't connect any equipment to the POTS without having BABT approval.
One certainly CAN. It's whether one should which is the main issue ;)

hippy
Posts: 6096
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: Domestic Caller ID Revisited

Fri Aug 03, 2018 12:52 pm

RaspISteve wrote:
Thu Aug 02, 2018 10:07 pm
The problem with Caller ID signaling is that it appears before the first ring cycle which means the modem has to be ON-HOOK and line therefore open to a new call.
But caller ID systems are designed to detect the number calling, then not allow the ring to come through if blacklisted, or not whitelisted.

I cannot see what the issue is with what you describe; it is how it works.

What are you finding a BT or third party supplied Caller ID Blocker will not do which requires having to build your own system ?

RaspISteve
Posts: 98
Joined: Wed Oct 24, 2012 9:15 pm
Location: Cheltenham, UK

Re: Domestic Caller ID Revisited

Fri Aug 03, 2018 5:36 pm

hippy wrote:
Fri Aug 03, 2018 12:52 pm
RaspISteve wrote:
Thu Aug 02, 2018 10:07 pm
The problem with Caller ID signaling is that it appears before the first ring cycle which means the modem has to be ON-HOOK and line therefore open to a new call.
But caller ID systems are designed to detect the number calling, then not allow the ring to come through if blacklisted, or not whitelisted.
I know this. I was looking at how it could be done without a bought Caller ID System. My point was that using a 'standard' modem won''t work because they generally need to go OFF-HOOK to do their thing.
hippy wrote:
Fri Aug 03, 2018 12:52 pm
I cannot see what the issue is with what you describe; it is how it works.

What are you finding a BT or third party supplied Caller ID Blocker will not do which requires having to build your own system ?
I know this too.

My DECT phone has a blocker feature but it's limited to 16 entries. I bought a Caller ID Blocker box from QVC for fifty quid and its PANTS (actually worst but its a family forum). Neither system allows enough actual control over what they do to make them useful. The blocker box does what it advertises but also randomly blocks calls from my lady friend and my daughters. When they do get through I end having to apologies and explain. This makes the little box a bit useless and got me thinking about how I would do it. The little box is now back in its box.

Given the reported problems with some/most modems I looked elsewhere for a solution. It was mostly an intellectual problem for me to solve which I think I did and thought to share my sources so that others could follow in their own way perhaps in locations where connection to a PSTN is less problematic/punitive.

I also have to say that Dougie is right. Connecting non-approved kit to a UK phone line gets one on the naughty list and I cannot recommend this.
gkaiseril wrote:
Fri Aug 03, 2018 1:42 am
You will need some circuitry to tap onto the POTS line and decode the FSK signal into the 16 possible characters. In the beginning this was done with a modem that connected to the POTS line and then fed the signal to the computer via a an RS-232 cable.
The UK Caller ID system for those that didn't read up on the two PDFs I listed uses a V.23/1200 bauded asynchronous byte stream starting with an ID byte then a record length byte, a multiple record payload and a checksum byte. The multiple payload records contain byte data and ASCII coded text characters including date/time detail, caller number as ASCII digits and other ASCII text describing the associated call.
Share and Enjoy.

drgeoff
Posts: 9901
Joined: Wed Jan 25, 2012 6:39 pm

Re: Domestic Caller ID Revisited

Fri Aug 03, 2018 6:46 pm

For the benefit of non-UK readers and possibly some in the UK who have a PSTN line type presentation from a provider other than BT.

There is no single universal Caller ID system. For example the US system uses Bell modem tones (not V.23) sent between first and second ring. Some countries use DTMF tones rather than modem ones. Some countries throw a line polarity reversal into the mix too.

And the UK is not the only part of the world with approval regulations.

Return to “Other projects”