" TTY stands for Text Telephone. It is also sometimes called a TDD, or Telecommunication Device for the Deaf. TTY is the more widely accepted term, however, as TTYs are used by many people, not just people who are deaf.
A TTY is a special device that lets people who are deaf, hard of hearing, or speech-impaired use the telephone to communicate, by allowing them to type messages back and forth to one another instead of talking and listening. A TTY is required at both ends of the conversation in order to communicate."
-AboutTTY.com