telecommunications devices for the deaf

A telecommunications device for the deaf (TDD) is an electronic device for text communication via a telephone line, used when one or more of the parties has hearing or speech difficulties. Other names for TDD include TTY (telephone typewriter or teletypewriter, although TTY is also a term used for teletypes in general), textphone (Common in Europe and the UK), and minicom (United Kingdom).
See more at Wikipedia.org...