A new modem standard provides reliable in-band transmission of TTY/TDD signals via the speech channel of all digital cellular phone systems. The standard includes error correction such as Forward Error Correction (FEC), and also performs interleaving to spread each TTY/TDD character over several adjacent speech frames. The modem standard can be applied to all speech coding standards and all mobile phone systems. The standard can be implemented using a special interface cable or box between a conventional TTY/TDD device and a mobile phone. The interface can also be integrated directly into new mobile phones, and necessary signal processing can be performed by the mobile's DSP. A new Universal TTY/TDD device can also be provided that supports both modem standards: conventional Baudot code as well as the new modem signals, and implemented using specialized hardware, or using conventional hardware such as a laptop computer with specialized software.