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I am working with alarm signalling systems at the moment, and they communicate with 2400bps v22bis modems.

I would like to be able to intercept the traffic on the phone line as a proof of concept.

The only device/software I can find to do this is the 3am Linescope.

This appears to be a phone line to audio adapter, connected to a sound card with some software. It is ~€500 so is quite costly.

As far as I understand it, technically intercepting v22bis traffic is possible, it just looks like it is not a common task.

Does anyone know of any cheaper or free alternatives?

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    You are right about it not being a common task. It's been YEARS, but all equipment I know of is expensive and specialized. This is such a specialized question, you might not get a lot of responses. Maybe calling your local telco engineer? Phreaking forums?
    – schroeder
    Jul 2, 2014 at 15:54

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Free software to demodulate the signal exists, see http://bellard.org/linmodem.html

For the physical signal acquisition on first approximation I'd use a digital oscilloscope, something like https://www.sparkfun.com/products/11219 but it should be possible to do it a lot cheaper unless you plan to reuse it for other projects.

If you can cut the line two modems should be usable, and the modems could be gotten for cheap, with your computer in the middle.

If you can't even strip the phone cable a http://en.wikipedia.org/wiki/Hall_effect_sensor is the way to go I'd say, in conjunction with the digital oscilloscope.

Interesting project, I would love to read more about it if you succeed.

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