I'd like to be able to interract with a piece of hardware which is able to be controlled by a standard phone line (and provide vocal metrics, ...)

Let me stress that it is not a modem, nor has it modem capabilities. I'm playing with plain old DTMF and pre-recorded audio messages.

What I want to achieve is :

  • Intercept phone calls triggerd by the device
  • Trigger a phone call to the device

And in both case being able to detect/send DTMF and record audio messages on the PI.

It has it's own power source so I suppose it won't need much of the power provided by a normal phone line. Thus I hope it will work with GPIO power...

Do you have any ideas on how to achieve this ?

  • Do you want the Pi to act like a telephone and be able to "call" and "answer calls from" this device, or do you want to "tap" the line it's connected to and listen in?
    – TomG
    Oct 4, 2014 at 19:03
  • Using an old modem with a DTMF decoder built in is actually exactly what you want. 3COM used to have a modem with every feature you need, it had a built in DTMF decoder and could also digitally record rec'd over the phone line.
    – Tyson
    Oct 5, 2014 at 2:28
  • @TomG I want it to act like a telephone. I don't want to tap on it. One reason is the fact that there is no real phone line availiable.
    – Cerber
    Oct 5, 2014 at 7:06
  • @Tyson I wasn't able to find a suitable modem : I've found out about linux.die.net/man/8/vgetty but all voice-modems I found where PCI cards which doesn't help when you have a PI
    – Cerber
    Oct 5, 2014 at 7:20
  • I was heading down the path of using Asterisk with a USB device to connect to the phone line, but there don't seem to be many options at the low end.
    – TomG
    Oct 6, 2014 at 1:21

1 Answer 1


There are many solutions to this hardware selection problem.

I would suggest this specific google search term RS232 modem voice DTMF.

Using that search term I saw a number of choices, including this one that seems to have the additional advantage of operating on 5V.

While you may not think that a "MOdulator DEModulator" (i.e. MODEM) is what you want, it is. You just don't want the computer to computer over telephone line communication convention. However you certainly do what to modulate and demodulate analog to digital, which is what a modem does.

For interface to the pi, you might want to use this or something like it.

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