updated
I used a terminal app kermit to talk directly to the modem; dial strings must be terminated by ; to make them work:
purism@pureos:~$ sudo kermit ~purism/.kermrc
C-Kermit 9.0.305 OPEN SOURCE: Alpha.02, 19 Sep 2020, for Linux+SSL+KRB5 (64-bit)
Copyright (C) 1985, 2020,
Trustees of Columbia University in the City of New York.
Type ? or HELP for help.
(/home/purism/) C-Kermit>c
Connecting to /dev/ttyUSB3, speed 9600
Escape character: Ctrl-U (ASCII 21, NAK): enabled
Type the escape character followed by C to get back,
or followed by ? to see other options.
----------------------------------------------------
atd##62#;
OK
^MODE: 3
+DISC: 5,0,0,31,"##62#",129
NO CARRIER
^MODE: 9
...
it also does work to dial a valid number:
atd0170xxxxxxxxx;
NO CARRIER
^MODE: 3
+DISC: 15,0,1,69,"0170xxxxxx",129
^MODE: 9
NO CARRIER
^MODE: 3
+DISC: 15,0,1,69,"0170xxxxxxxx",129
^MODE: 9