CSD data calls are perfectly possible in GSM - your carrier can disallow them though. However, you usually feed and receive data to/from the modem in a differen

CSD data calls are perfectly possible in GSM - your carrier can disallow them though. However, you usually feed and receive data to/from the modem in a different way than via its audio interface ;) Other than that, you're right. It's possible to come up with a way to pass data through GSM codec, but it definitely won't be a regular FAX connection then.