TTY stands for “teletypewriter.” It was initially developed for people with hearing or speech difficulties to communicate over traditional telephone networks. TTY devices translate typed text into electronic signals sent across the telephone network and retranslated to text at the other end.

The proper notation is TTY (teletypewriter) and it doesn't matter whether you are using an older model (teletypewriter) or the newer model (telecommunication device for the deaf-abbreviated as TDD).

TTY is the acronym for Teletypewriter and is also referred to as a TDD, which is the acronym for Telecommunications Device for the Deaf. TTY is currently preferred. • When calling TTY users, let the phone ring at least 10 or more times before hanging up.