ModemManager by default uses 57600bps as baudrate when opening TTYs for probing. This works most of the time because baudrate autodetection makes it just work. For some RS232 modems, though, explicit baudrate settings may need to be applied, if the modem is known to work in a baudrate different to 57600.
https://lists.freedesktop.org/archives/modemmanager-devel/2017-March/004075.html
Pushed as 11a26f10668659fb0c043ec9bc88debacce8cab1 to git master
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