Android offers support for TTY, which can mean "teletypewriter" or "text telephone" among other things. TTY is a communication tool allowing text communication over standard phone line connections, by converting the text input to audio and then decoding the audio back into text for reception. You can make a TTY call from your Droid to a TTY box on a land line, or receive a call in the same fashion. Since the 1960s, TTYs have been used by hearing-impaired individuals, among others, for telephone communication.
TTY services were originally developed for use in the news field where rapid text communication was necessary, but in the 1960s the technology was adapted for use over standard phone lines. A TTY machine can send and receive text information over a telephone connection to allow hearing-impaired people to communicate. Sometimes called a text telephone (TT) or a telecommunication device for the deaf (TDD), the term TTY covers all the different types available, according to the organisation Telecommunications for the Deaf.
To activate TTY calling on a Droid phone, open the application list and find "Settings." Tap on the icon. Scroll to "Call Settings" and tap. Tap "TTY Mode" and then select the TTY mode you want from the list. Tap "Home" when you're done. TTY modes are: "TTY Off;" "TTY Full;" "TTY HCO;" and "TTY VCO." You should only use the TTY settings if you are going to connect your Droid to a TTY device. Otherwise, use text messaging and e-mail options for text communication since the TTY interface is specialised for transmitting text over audio signal through traditional phone lines.
The variants of TTY are Full, HCO and VCO. Full TTY means that there is text-only communication on both sides of the phone call. HCO stands for "Hearing Carry-Over" which means that you hear a voice read the incoming text and you type the outgoing text. VCO stands for "Voice Carry-Over" which means that you speak the outgoing text and receive a text message back as your incoming communication. HCO settings are typically amplified for the hard of hearing. Use caution with headsets and hands-free ear pieces when enabling HCO because of this increased volume.
The TTY technology is one way at a time, meaning that only one side of the conversation can be transmitted at a time. To facilitate easy back and forth communication, users of TTY have developed some shorthand. "GA" indicates that you should "go ahead" and type. It functions much like saying "Over" in radio communication, indicating that you have stopped and are waiting for a response. When placing a call, wait at least seven rings for an answer, since a hearing-impaired person may take longer to notice an incoming call. A typical greeting when a hearing-impaired person answers is "GA." It's polite to then respond with your name and the reason you called. End a call with "SK" to indicate that you've "stopped keying" on your side.