I’m trying to find an inexpensive way to access a remote device interactively. A low bit rate (300bps) is fine.
The GM86x modules support CSD but it looks like carriers have been phasing this out. Before purchasing a T-Mobile pay as you go SIM, a call to their support line confirmed CSD was supported. After failed attempts trying to originate a CSD call , T-Mobile support now indicates CSD is no longer supported.
I was able to use an old modem configured for Bell 103 / AFSK encoding connected through the analog headset interface on the Telit module to make a voice call to another modem connected to the PSTN. This works, but it’s a little ugly.
Am I missing something simple here? A data plan would make this easier but too expensive.
If this is really the only route to inexpensive data calls over the cell network I’ll probably end up using something like an old TCM3105 running in V.23 mode which should be able to talk to a standard off the shelf modem.