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I am wondering if you can interface a standard, RJ11 telephone through GPIO, and input an output through the phone, even using asterisk to connect to voip. The phone I have is a GE model number 29255ge2 which I believe was made around the late 1990s and early 2000s. I am wondering if you can remove the RJ connector from one end, somehow connect the red and green wire to gpio. One concern I have is that 5 volts will be enough to power the phone, and also making it ring, I know normal ringing voltage is 60v AC I believe.

BTW I am not connecting this to the phone line, also basically if you are misunderstanding this I am basically trying to use the pi as cheapo Analog Telephone Adapter.

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    The Pi's GPIO are 3V3 and can drive perhaps 20 mA. What signals do you plan to send down the phone line? Would connecting non-approved kit to the phone line be legal in your jurisdiction? – joan Oct 04 '15 at 22:16
  • Not to mention what is on the other end some phone lines send a very high voltage for the ring - which would instantly fry your Pi. What is the actuial goal? beyond connecting a phone to the Pi. – Steve Robillard Oct 04 '15 at 22:22
  • I am not connecting anything to the phone line, I am connecting a phone to the pi, and basically I want to be able to make calls from it, its kind of a "because I can" thing and I don't care abount incoming ring that much. In fact, one of the reasons is because I don't even have a landline. –  Oct 04 '15 at 22:51

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So from what I understand you're trying to make the RPi into an ATA device. Some combination of GPIO pins are wired into a phone cable that plugs into a analogue phone handset.

Could be possible in a run-around way, with a DAC/amp circuit in-between, you could use I2C for audio i/o see this question for more info.

A couple of other options might save some hair pulling:

  1. Get a USB to FXS device like this - the one I linked has Linux drivers, so some possibility of an easy-ish job, then combine with something like RasPBX running on the RPi.
  2. Ditch the idea of using the RPi and just buy a cheap or used Cisco ATA and plug that into your analogue phone handset.

Personally I'd go with the second option and save myself about 50 hours.

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