Some cell phones are TTY/TDD (telecommunications device for the deaf) compatible, meaning a terminal can be plugged into the headset jack for text communications. Is it just a hidden RS-232 interface?
TTY/TDD uses tones in the audio band (similar but not compatible with the old Hayes modems) to communicate over telephone lines. The original protocol uses Baudot code at 45.5 or 50 baud. In 1994 the ITU approved a newer V.18 standard.
So I would presume the headset jack would just pass these tones back and forth to a TTY/TDD modem. It would not be a hidden RS-232 interface -- i.e. the cellphone is not acting as the modem.