OK, so here’s what I figured out.
I couldn’t get smstools3 (smsd) to do anything, but it did lead me to search for the Quectel AT command documentation for the modem: https://www.quectel.com/content/uploads/2021/03/Quectel_EP06EG06EM06_AT_Commands_Manual_V1.0.pdf
You can use the socat command line program to send AT commands to the modem. socat is already installed on the Omnia.
You need to know the device for the modem. A grep of dmesg messages shows the modem installed four devices:
root@turris:~# dmesg | grep USB
...
[ 16.263785] usbserial: USB Serial support registered for GSM modem (1-port)
[ 16.277544] usb 1-1: GSM modem (1-port) converter now attached to ttyUSB0
[ 16.291292] usb 1-1: GSM modem (1-port) converter now attached to ttyUSB1
[ 16.308579] usb 1-1: GSM modem (1-port) converter now attached to ttyUSB2
[ 16.321956] usb 1-1: GSM modem (1-port) converter now attached to ttyUSB3
[ 16.336198] usbserial: USB Serial support registered for Qualcomm USB modem
...
I know that /dev/ttyUSB2 is the device being used for my mobile broadband. I tried them all, but only /dev/ttyUSB3 worked:
root@turris:~# socat - /dev/ttyUSB3,crnl
At this point you can type AT commands.
To put the modem into text (rather than “PDU”) mode for SMS messages:
AT+CMGF=1
To list all all the SMS messages:
AT+CMGL
This lists only unread messages, and marks them as read, so if you run it again, the messages won’t appear. To see the read messages again:
AT+CMGL="REC READ"
To delete all the messages:
AT+CMGD=0,1
For me, some messages were in plain text and others were encoded. The encoded messages were simply using a four-byte hex value for each character, e.g. “0048” = “H”
To decode these, I used the xxd program (on another machine, it wasn’t installed on the Omnia):
# echo -n '00480065007200652019007300200079006F007500720020006F006E0065002000740069006D0065002000700061007300730063006F00640065003A0020003700300031003500330039000A000A0054006800690073002000690073002000760061006C0069006400200066006F0072002000330020006D0069006E0075007400650073002E' | xxd -r -p
Here s your one time passcode: 701539
So not exactly the most user-friendly process, but it gets the job done.