retpolanne blog

Your friendly programmer catgirl 🏳️‍⚧️😺

16 September 2022

making a call blocker

by Anne Macedo

So, my dad still have a landline, in 2022. And it’s being spammed with robocalls.

It’s so annoying that I decided to use a Raspberry Pi as a call blocker. For that, I need to play with an USB modem!

Call blocking solutions

I’ll keep them out of scope to be honest, since they are their environment on their own. Links for the tool that I want to use is on [1].

To make it work, I figured out I needed:

You’ll have something like this (I hope Jekyll doesn’t break it :( )

  phone      --------- |
                       |---  splitter --- my isp router
  usb modem  --------- | 

Now that I have everything, here’s the problems.

Caller ID… doesn’t work!

In order to check if the telephone is ringing, you need a tool with a very interesting name… a name that Brazilians can’t read without laughing.

sudo apt install cu

That’s the name of the tool, cu.

Anyways, I recommend running cu on tmux, since it holds your tty. You can’t exit cu.

Also, I needed to change my user to the dialout group.

sudo usermod -aG dialout pinkiepie

And them start cu attached to /dev/ttyACM0. By making a call, it should show up the RING and the caller id, right?

cu -l /dev/ttyACM0



No caller id :(

I need to figure out two things:

I’ve bought a DTMF to FSK converter anyways. :money-with-wings:

And also need to find my modem’s chipset.

Finding the modem’s chipset

You can basically run lsusb to get the VID:PID, but you can get the chipset model from cu

cu -l /dev/ttyACM0 

Found out about this on [2].

lsusb tells me it’s a Conexant. From the Trendnet Tfm-561U datasheet, it says it supports DTMF.

I tried enabling VCID, and it says OK, but nothing shows when it rings.



On the same forum [2] someone suggested using ATDT for dialing. This guide [3] teaches how to do this.

It’s basically like this:


It says ok, but doesn’t ring my cellphone.

Ringing without DDD works!

ATDT xxxxxxxxx

It was nice messing up with this, but the DTMF/FSK converter should arrive on monday. Hopefully no one will call this weekend.

To be continued.


[1] Call Attendant GitHub [2] Need some guidance on setting up Caller ID Feature [3] 11. Trying Out Your Modem (Dialing Out)