Caller ID / Phone interfacing

I've been reading about how Caller ID works and think it would be a fun project to try and decode it myself. I already understand a lot about digital circuits, so I think once I had the two tones processed into a logic value, the rest would be easy. Particularly since it all seems to be using the bell 202 modem standard. I think it would be a good learning experience for audio decoding and such especially, since I'm also interested in radio.

The thing I'm wondering about though is how one actually interfaces with the telephone line. I know a lot of voltage comes through there, up to around 100v AC for the ring, and like -48v DC or something when on-hook, then maybe -8V DC off-hook. But all at a very low milliamps. The thing I'm not quite sure about though is how the modulated Caller ID signal comes though the line.

You can't pick the phone up to hear the modem tones, because that takes it off-hook of course. So how exactly does something "listen" to the phone line without taking it off-hook? Are the tones modulated as AC into the -48V DC somehow? I've also read about what seems to refer to using a normal 1200 baud modem somehow to process the Caller ID data. I just don't get how this works. As far as I know, a modem has to go off-hook before it can process tones.

Relatedly, when thinking about that, it made me wonder if it's possible to listen to an active call on another extension in the house without taking the line you're connected to off-hook, too. I know that that treads into snooping territory, but I just like to know how the phone system works in general. I always thought you had to go "off-hook" to hear the line.

Once I understand more about radio building too, I think combining this knowledge of Bell 202 would be fun to receive any possible data being sent out there as well. And/or to do my own short-range experiments.

Anyhow, anything anyone can shed on any of this would be much appreciated!

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The answers are

To monitor the phone line you need a high input impedance cirucit. With input impedances greater than 10Kohms AC

The monitor circuit should be AC coupled

have a blocking circuit that blocks the high voltages

The caller ID monitor should decode the BEL202 modem tones (1200baud).

There are plenty of circuits on the web


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Joe G (Home)

You can always just buy the chip to do this:

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You may not learn much about the inner workings of Caller ID and call progress tones, but you'll sure get an education in how to get something done quickly :) That alone is worth something. :)

Good luck with your project!


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This was discussed with plenty of technical detail in this NG before.

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