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.
If you don't have a TTY, you can still call a person who is deaf, hard of hearing, or speech-impaired by using the Telecommunications Relay Service (TRS). With TRS, a special operator types whatever you say so that the person you are calling can read your words on his or her TTY display. He or she will type back a response, which the TRS operator will read aloud for you to hear over the phone. Toll free TRS services are available 24 hours a day, 365 days a year.