I’d like to record some robo-calls coming into a landline. At first I thought an old standalone answering machine would work. But the robo-call seems to be able to detect that & doesn’t leave a message. Does anyone know how the wiring is setup on analog lines? If I can modify a phone cable to tap the audio signal then I could answer the phone and record the audio. I’m guessing that signal might need some adjusting to connect to a line level input.
First you need to know if your phone line is really analog or not … if the handset plugs into your internet router, then, even though the handset is the same, it’s no longer analog.
Before getting into the details of interfacing with PSTN lines, have your tried the simplest approach - put a contact mic onto the phone receiver (i.e. just record acoustically from the earpiece of the handset)?
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I had considered and rejected a regular mic, but hadn’t even thought about a contact mic. Good idea.
Yeah, I’m 100% sure it is analog.
If it is indeed a landline, I did this in the 90s. It consisted of an actual telephone which I patched via the input and output jacks into an audio interface via the mic and speaker in the handset.
I think I had to amplify the phone output, and that I cannibalized a Radio Shack 3-inch amplified speaker to do it.
Or this, as mentioned:Telephone Recording Pickup Coil Suction Cup w/5' cord,New | eBay
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Google “VEC TRX-20 Direct Connect Telephone Record Adapter”
Thanks. That looks like just the right tool for the job. I knew someone here would have some practical telephony experience.
Phone lines are not just audio. They also provide power to ring the phone. You will get shocked pretty hard if holding the wires when a call comes in. Some sort of premade adapter would be the best option, something like what was commented above. I use older Clearone converge pro units to record phone for interviews on a regular basis. They can be found on Ebay for cheap. It’s an advanced option, you need the software to program it. You have to set up the matrix for inputs to outputs sort of like setting up Dante and also any processing chain you want inline for incoming and outgoing audio. It’s a very in-depth solution and if you have not used them before it wouldn’t be an off the shelf start recording kind of thing but if you learn how to program it the final results are about the best you can get from a phone line. You would also set a microphone up for the input so it can be answered like any other phone. Many units can be chained together and dialed separately so you could conduct and record interviews with many people at once, sending a mix minus back to each caller so they on the far end hear everyone except themselves.
The (usually curly) cable between the base and the handset is, though. That’s what the $10 adapter mentioned above does. Not for professional use, but just to record the audio of an incoming call, such as robocalls.
When I did this, I connected my jack to the internal connections inside the phone.
Even when the connections were wrong though, the voltage from the ring was low amperage, even though it’s around 40 volts, and no harm came to the old Apple IIsi computer that was connected to that mess.
I suppose it’s worth mentioning that connecting directly to PSTN lines is also illegal in many countries, so anyone considering doing this should perhaps check with suppliers of radio station equipment. That’s where you’ll get a fully-licensed device such as the type ProductionPlace describes above.
For the rest of us, cheap-and-cheerful handset adapters will work fine, but don’t expect to be able to do phone interviews for podcasts, etc. Interestingly, there are a range of new products for just that – basically a small USB mixer with a Bluetooth interface to one channel – which sprang up like mushrooms during the pandemic (e.g. Zoom PodTrak).
Mostly aiming for sampling fodder from the current robocall selections
Yikes! I can’t believe (OK really I can) how dumb I’ve been overlooking the obvious.
I have a phone with a headset jack on it.