MMFWD

Forward calls and texts using ModemManager framework.

This is a daemon that forwards calls and text messages via email and SIP(planned?). I made this python program to receive KYC messages whilst I'm overseas. South Korea is a weird county where all KYC verifications are done via personal mobile number. Without a mobile number, doing anything online is almost impossible. The daemon can be run on a small and reliable device that is kept somewhere safe.

https://pygobject.gnome.org/getting_started.html
https://code.nap.av.it.pt/mobility-networks/modemmanager/-/tree/1.18.12/examples/modem-watcher-python

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Call answering machine (mmfwd-callam)

https://github.com/user-attachments/assets/b4782fbf-cf7d-4dba-8e81-26a8a6e084a0

https://github.com/user-attachments/assets/15a80630-4a53-4dc5-8954-4366e0f7c6d0

Usage:

mmfwd-callam <modem audio char path> <output prefix>

Making audio samples:

ffmpeg -i FILE -vn -ar 16000 -ac 1 -f s16le OUTFILE.pcm

or use Audacity:

Go to File - Export Audio
Format: "Other uncompressed files"
Channel: Mono
Sample Rate: 16000 Hz
Header: RAW (header-less)
Encoding: Signed 16-bit PCM

Audacity overrides the extension to .raw. Change it to .pcm.

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The audio streaming is handled by the child process mmfwd-callam since the audio serial port is synchronous(CTS/RTS). Common tools like aplay and arecord do not support nonblocking IO.

src/mmfwd-callam.cpp:

void init_params (void) {
    params.hello_sample_pat = "hello/*.pcm";

Place PCM files in hello/*.pcm. Then the main mmfwd Python module can spawn the process.

src/mmfwd/init.py:

now = datetime.datetime.now(datetime.UTC)

n_from = call.get_number() or ""

dir = "rec/%02d-%02d" % (now.year, now.month)
filename = now.isoformat(timespec = 'milliseconds') + '_' + n_from
path = dir + '/' + filename

os.makedirs(dir, exist_ok = True)

exec = [ ud.instance.callam['exec'], "/dev/" + ud.audio_port, path ]
ud.instance.callam_proc = subprocess.Popen(exec)
# 5 minutes timeout
ud.instance.callam_timer = GLib.timeout_add_seconds(
    60 * 4,
    self.on_call_timeout,
    ud)

A pair of recorded audio files will be placed in rec/YYYY-MM/<ISOTIME in millis resolutiono>_<NUMBER>.<INOUT> where

NUMBER is the phone number of the calling party
INOUT is "in" for the audio from the calling party and "out" for from mmfwd

To encode the recorded raw audio files:

ffmpeg -ar 16000 -ac 1 -f s16le -i PCM_FILE -c:a libopus OUTFILE.ogg

Local playback

Config to enable. 1 for incoming audio only, 2 for outgoing audio only, 3 for both. 0 to disable:

mmfwd:
  instances:
    -
...
      call-am:
...
        playback: 1
...

Enable ringtone (optional)

https://github.com/user-attachments/assets/209f5bdd-2e63-4373-81d2-5a47c403db82

Make a simple ringtone:

# short bursts of alternating 250 and 500 Hz sine waves
truncate -s 0 bell.pcm
for (( i = 0; i < 6; i += 1 ))
do
  ffmpeg -loglevel error -f lavfi -i "sine=frequency=250:duration=0.1" -f s16le -ar 16000 -ac 1 - >> bell.pcm
  ffmpeg -loglevel error -f lavfi -i "sine=frequency=500:duration=0.2" -f s16le -ar 16000 -ac 1 - >> bell.pcm
done

# encode it to opus
ffmpeg -loglevel error -f s16le -ar 16000 -ac 1 -i bell.pcm -c:a libopus ringtone.ogg

Config to enable ringtone:

        ringtone-exec:
          - ffplay
          - -loglevel
          - error
          - -autoexit
          - -nodisp
          - ringtone.ogg
...

MM Patches

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/1293
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/996

Another idea (070 number)

Major telcos in South Korea(KT, SKT, LG U+) offer VoIP services. After subscribing to such services, the SIP credentials can be extracted from the device. AFAIK, the telcos' SIP servers don't do whitelist connections so the credentials can be used pretty much from anywhere with any SIP+VoIP client implementations.

Sources:

https://blog.naver.com/rehearsal/80099885977
https://www.clien.net/service/board/use/1219343
https://koreapyj.dcmys.kr/152

ModemManager and LTE modems such as SIM7600 are usually optimised for mobile date network connectivity on Linux world, not for voice calls. You might as well get a 070 number(the standard VoIP prefix in Korea) and redirect the calls and messages to the SIM number to the VoIP number to get high quality audio straight from the telcos' IP core network.

Pro: Stable audio stream
Con: Adding another point of failure to the system