TTY stands for Text Telephone (or Teletypewriter for the Deaf). 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.
To use a TTY, you set a telephone handset onto special acoustic cups built into the TTY (some TTY models can be plugged directly into a telephone line). Then, type the message you want to send on the TTY's keyboard. As you type, the message is sent over the phone line, just like your voice would be sent over the phone line if you talked. You can read the other person's response on the TTY's text display.
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.