@ Varthall
The STFax logfile says that it's opened usbmodem.device, but given that any activity on said device locks up the program, I'm not sure if it's been successful.
I'll give Term a try, but it'll have to wait until later. It sounds like a good diagnostic, though.
The phone is an LG CU515. According to the documentation I can find, it definitely supports faxes. There's a data transfer mode setting that toggles it between massstorage.class, cdcacm.class, and some proprietary format, but I'll have to look more carefully for additional fax settings.
Thanks for the pointers.