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. VCO is a technology developed that combines voice and text communications over the telephone. VCO allows people to speak directly to the person they are calling, then read that person's response on their VCO or TTY phone display. The other person's voice responses are typed for you to read, either by the person himself or by a special operator through a Telecommmunication Relay Service (TRS).