# How to build a telephone answering machine with caller id

I'm brand new to electronics. I'm interested in building my own telephone answering machine with caller id receiver from scratch as a learning project. How would you suggest I get started?

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Not a real question. It should be closed. – Leon Heller May 20 '12 at 19:09
I suggest you start by digging up and reading a detailed description of the signalling protocol used to send the caller ID. – Olin Lathrop May 20 '12 at 19:55
It's better if you design something and then if you get stuck with something not working, ask a concrete question with a schematic and a description of what isn't happening and what should be happening. – Kaz May 20 '12 at 20:17

## 1 Answer

The operation of such a device is really quite simple. Take a look at the description of ATA cards and the Asterisk Project.

You need three components: A telephone interface. A computer with some DSP like functionality, this might be some sort of microcontroller with fixed point math ability. A user interface, a combination of software and hardware to interface to the above.

The caller id (CID) functions differently in each country. But, generally it is a low baudrate FSK (frequency shift keying) signal sent between the first and second ring. You don't hear it simply because the receiving phone is on hook, however it can be recovered with the correct electronics.

Your DSP can demodulate the FSK into baseband and find the CID signal within that.

Technically in most countries, interfacing any electronics to the telepone network is against the rules, though there is little that can be done to detect you are playing with the phone system.

WARNING: During ring-in there is >50V AC present on the line. If you touch it it really hurts (I've been there) and can be dangerous.

There are many examples of telephone interface circuits of varying complexity available on the internet.

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 WARNING: During ring-in there is >50V AC present on the line. If you touch it it really hurts (I've been there) and can be dangerous. The voltage is also present during dial-tone. I learned this taking the modem out of a computer. It shocked the hell out of me, so I went to unplug the computer... it was already unplugged... then I realized the phone cable was still hooked to the modem – Earlz May 22 '12 at 6:09