Play around with https://github.com/quiet https://github.com/quiet or http://www.whence.com/minimodem/ http://www.whence.com/minimodem/ to see what the highest

Play around with https://github.com/quiet https://github.com/quiet or http://www.whence.com/minimodem/ http://www.whence.com/minimodem/ to see what the highest bitrate and ECC you can send over a GSM voice connection. Should be fairly quick.