A different kind of phone hacking

I previously mentioned this awesome project to rip out the innards of an old rotary dial phone, and stuffing a Raspberry Pi inside which plays a different MP3 file out of the earpiece each time you dial a different number.

I decided to have a go myself, having found 2 old phones in the loft. These are old British Post Office / GPO phones, so the wiring is probably different. I’d also like to keep these phones intact if I can, so I decided to do a bit of probing to see if I could attach a RaspberryPi to the phone’s existing internal terminals.

First – I had to choose a phone. The red one is type 706, the white a newer type 746.

I soon plumped for the white one as its terminals were more accessible and there’s more room inside.

The red one has an extra button and even has a vertical early PCB ‘card’ inside. Following the wires round inside is a nightmare, so I put this one back together and let it be.

I started probing – appropriately – with an old British Telecom multimeter, which was fine for locating the wires for the receiver ‘hook’ (it might be nice to have a dial tone when you lift the handset!), but a multimeter doesn’t react quickly enough to count pulses from the rotary dial. So I made a simple circuit with an LED to test the different terminals inside the phone to see if any were connected across the dial switches – and indeed they were. By breaking the LED circuit, and touching the loose ends across different pairs of terminals and dialling, I could see when the light flickered, showing the dial breaking the circuit to transmit the numbers.

Modern fixed phones dial by sending tones down the line to the exchange – known as DTMF or TouchTone. Old rotary phones worked by breaking a circuit to the exchange creating pulses at a rate of 10 pulses per second. 1 break in a second means you’ve dialed 1, 2 breaks is a 2 and so on. But what about 0?

I’m might confused by the wiring instructions in the original article, so next steps are to look carefully at the code, and perhaps write my own. And buy a jack plug so I don’t have to solder the earpiece on to the Pi direct.

Thanks to my former colleague Frank Bath for information about Strowger telephone dialling, and to James West for reminding me about phone phreaking and an old way you could sometimes cheat a payphone in the UK by tapping the cradle switches to mimic the dial pulses. I used to actually phone people this way on my home phone. Hey, I didn’t get out much, and phone numbers in my village were 4 (FOUR) digits long. And I can still remember my friends’ numbers: 2235 and 3516. And no, they’re not my PINs…


I’ve now wired the earpiece up…

…and I got as far as getting the Pi to detect when I hang up to stop the radio playing.

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