Connecting the pi to a phone using a RJ11 cable to send and receive audio

Question

I'm trying to connect the pi 4 model B to a phone that I bought off Amazon (https://www.amazon.ca/Wild-Wood-Phone-Retro-Design/dp/B00H20B2GM). This phone is not connected to a landline or anything as it's just sitting on my table for fun. I've gone through various tutorials and posts online. I'm really confused as I'm trying to play recorded audio files on the phone using the pi and also save audio on the pi using the phone handset. I think it would be a fun way to use these cool phones!

From what I've found so far, it looks like I might need a modem like this (https://www.usr.com/products/56k-dialup-modem/usr5637/) that I could use to send and receive audio using the pi to the phone. I need to connect the RJ11 cable to the Raspberry Pi and send and receive audio using that interface. Basically, I'm trying to play some audio on the phone when I lift the handset and then be able to record audio too! I think the USB might be too much.

Can someone please give me a few tips if any to help get this going? I'm new to all this so trying to figure it out. Thanks!

Answer 1

What you erroneously refer to as a "RJ11 cable" is in fact a 8P8C modular connector and CAT5/6 cable.

The 8P8C connector on many Pi models is used for Ethernet over twisted pair. This uses a low voltage differential signalling system.

RJ11 uses a similar six-position modular connector but only 2 pins are used for a 50V analog telephony interface.

Most telephones actually use a 4P4C modular connector.

While the connectors may be physically similar, they are totally incompatible so what you ask is impossible.

The Pi has NO analog capability, apart from a PWM audio output.

Answer 2

While a USB modem like that would give you the ability to interface between your RPI and a phone network, it's set up as a client and so won't generate enough power output onto the RJ11 cable to power a phone on its own.

I'm working on this as well and this answer on a similar question has been really helpful. It has a great breakdown of the current the phone requires and how to hook up the various functions of the phone. As for interfacing with the RJ11 socket, you can also buy an RJ11 socket breakout board from various shops like Pi Hut to interface with the connector more easily.

That all said, it might be easier to just crack open the phone itself and connect directly to the speaker and microphone wires inside. However, in all these options you will have an issue that the Raspberry Pi doesn't have analog audio-in capabilities and its analog output is limited to the audio jack. So you'll have to either get a USB audio adapter or an analog audio hat to interface with either the hacked RJ11 or the cracked open phone.