It actually is possible, but at very low bitrate. I've seen more than a few demo's showing at leats 9600 baud is possible. But you need someone on the other end to set up the gateway.

[edit to add] My memory was off, it was 300 baud. quick search yields things like this: http://www.gsm-modem.de/M2M/m2m-faq/how-to-send-data-over-gs... https://ieeexplore.ieee.org/document/1497979 https://github.com/quiet/quiet-lwip