Recording landline phone calls

Question

My cell phone is recording all phone calls (using Automatical Call Recorder on Android) and this has helped me a lot of times (both because I didn't hear something very clearly or as evidence for something).

Now I want to do the same for my landline phone (RJ11 connection, Belgian phone line). I have a Raspberry Pi 3 Mode B I received as a gift and haven't used yet. I was wondering if I could use my Raspberry Pi for this? In the ideal scenario it's recording all calls and storing them on my NAS.

I'm a programmer so if necessary I can code. I have little to none experience with building electronics.

Is this doable? If yes, where/how do I start?

Answer 1

My approach to this would be to do everything I possibly could to avoid interfacing the Pi directly with the phone line. It's definitely achievable, but it sounds a lot harder than I like my spare time.

Some Googling suggests that it shouldn't be too difficult or expensive to get hold of something that looks like this £16 adapter from Maplin. It accepts (I'm pretty sure) an RJ11 in from the phone socket, and outputs audio to a 3.5mm jack. That particular device, I note, is extremely badly reviewed - I suspect there may be better alternatives on Ebay.

Now you have to get the audio into the Pi. I'd recommend a USB soundcard.

Now all we have to do is listen to the audio input and trigger a recording if the incoming amplitude peaks over a threshold. There's currently a really good writeup on using SoX to do exactly this at pibat.afraidofsunlight.co.uk:

By specifying the right parameters to SoX’s silence command, sound can be monitored effectively trimming all noise until noise is detected, and then trim everything after so many seconds of silence.

This can be achieved with the following command:

rec - c1 -r 192000 record.wav silence 1 0.1 1% 1 5.0 1% : newfile : restart