Not sure if this is useful to off-gridders but I guess if you are in an area with voice towers that do not offer data, this could be useful.
I wonder if a software fax app on a GSM phone would be able to send faxes without needing data service. Or in fact, might it have to? Otherwise I guess if it were to use mobile data it would have to run over VOIP, perhaps with sketchier results.
I suspect you might run into issues with the voice data-compression used by modern cellular networks.
If that’s a lossy compression, then yeah that would surely blow the fax over voice GSM option. Although in principle a well-designed system would detect the fax/modem handshake and disable compression.
(edit) so are you saying CSD has been scrapped? Because IIUC CSD makes specific accommodations for data over voice.
