How to send

[htdig2 htdig2]

Hi guys,

with a specific USB GSM/UMTS modem (details below) running
  mmcli -m 0 --3gpp-ussd-initiate='*100#'
times out. Debug Log just says
  ModemManager[1879]: <debug> [1531392387.451747] [mm-port-serial.c:1288] mm_port_serial_open(): (ttyUSB2) device open count is 2 (open)
  ModemManager[1879]: <debug> [1531392387.464272] [mm-port-serial-at.c:459] debug_log(): (ttyUSB2): --> 'AT+CUSD=1,"002A0031003000300023",15<CR>'
  ModemManager[1879]: <debug> [1531392387.535640] [mm-port-serial-at.c:459] debug_log(): (ttyUSB2): <-- '<CR><LF>OK<CR><LF>'
  ModemManager[1879]: <debug> [1531392387.545907] [mm-port-serial.c:1345] _close_internal(): (ttyUSB2) device open count is 1 (close)
and that's it. No answer.

In minicom manually typing
  AT+CUSD=1,"*100#",15
gives back an answer, using http://smstools3.kekekasvi.com/topic.php?id=288 to convert it from UCS2 the answer turned out to be correct.

Seems it's one of the modems that requires USSD command strings to be non-encoded. I started digging around in the current code online and was hoping to find, maybe, some hint attribute that can be put into udev rules for telling MM not to encode USSD commands for this specific modem. Unfortunately I got lost when searching for mm_modem_3gpp_ussd_initiate[_sync]() in the section where MmGdbusModem3gppUssd seems to get (auto?)generated.

Question: Is it possible to tell MM to send non-encoded USSD command strings to the modem? If so, how?


Details:
- Raspbian Stretch, basically that's current Debian stable "Stretch" with a additions specific to Raspberry Pi
- package modemmanager 1.6.4-1, unmodified from the distribution
- lsusb: Bus 001 Device 007: ID 19d2:0039 ZTE WCDMA Technologies MSM

/org/freedesktop/ModemManager1/Modem/0 (device id '9cb4bd518f0e1b3c26cdfe3e3d7844a4f133a1cf')
  -------------------------
  Hardware |   manufacturer: 'ZTE CORPORATION'
           |          model: 'MF190'
           |       revision: 'BD_MF190TLF3V1.0.0B05'
           |      supported: 'gsm-umts'
           |        current: 'gsm-umts'
           |   equipment id: 'xxxxxxxxxxxxxxx'
  -------------------------
  System   |         device: '/sys/devices/platform/soc/20980000.usb/usb1/1-1/1-1.2'
           |        drivers: 'option1'
           |         plugin: 'ZTE'
           |   primary port: 'ttyUSB1'
           |          ports: 'ttyUSB0 (qcdm), ttyUSB1 (at)'
  -------------------------
  Numbers  |           own : 'xxxxxxxxxxxxxxx'
  -------------------------
  Status   |           lock: 'none'
           | unlock retries: 'sim-pin (3), sim-puk (10)'
           |          state: 'registered'
           |    power state: 'on'
           |    access tech: 'umts'
           | signal quality: '100' (recent)
  -------------------------
  Modes    |      supported: 'allowed: 2g; preferred: none
           |                  allowed: 3g; preferred: none
           |                  allowed: 2g, 3g; preferred: none
           |                  allowed: 2g, 3g; preferred: 2g
           |                  allowed: 2g, 3g; preferred: 3g'
           |        current: 'allowed: any; preferred: none'
  -------------------------
  Bands    |      supported: 'unknown'
           |        current: 'unknown'
  -------------------------
  IP       |      supported: 'ipv4, ipv6'
  -------------------------
  3GPP     |           imei: 'xxxxxxxxxxxxxxx'
           |  enabled locks: 'none'
           |    operator id: '26201'
           |  operator name: 'Telekom.de'
           |   subscription: 'unknown'
           |   registration: 'home'
  -------------------------
  SIM      |           path: '/org/freedesktop/ModemManager1/SIM/0'

  -------------------------
  Bearers  |          paths: 'none'