TTY and TDD enable hearing-impaired and speech-impaired individuals to communicate over the telephone. TTY and TDD work in conjunction–there is no distinguishing difference.
TTY & TDD and its uses
TDD & TTY:
TDD, or Telecommunications Device for the Deaf, is a software program that enables deaf or mute individuals to communicate over the telephone with TTY devices.

TTY–or telephone typewriter, teletypewriter or text phone–is a device used by deaf or mute individuals to communicate over telephone lines. The device is a “QWERTY” keyboard that is similar to a computer’s but smaller. A screen attached to device shows words as they are being typed.

Function:
The TTY is connected to a phone line. The Direct Connect ensures that the phone cord is plugged directly into the phone jack. A flashing light alerts when there is an incoming call.

Technological Advances:
TTY/TDD is available as a portable device and is used in place of a land line. Some TTY/TDD devices can be attached to hearing aids.

Protocol:
Callers must take turns when sending messages. You cannot interrupt as in a normal conversation. To facilitate proper protocol, codes are used at the end of each transmission to let the other person know when it’s his turn.