LG VN251S Service Manual page 18

Assistance
Assistive Communication Devices
TTY (Text Telephony) and TDD (Telecommunications Device for the Deaf)
allow individuals who are deaf, hard of hearing, or have speech or language
disabilities to communicate by telephone.
When a user types his or her conversation on a TTY keyboard, it is
transmitted as tones through the telephone. Tones are received by the
other person's TTY, translated into text and displayed on the screen. In
order to use the TTY network, you must have a TTY-compatible phone and
be in the TTY mode to place or receive calls. Note that most digital wireless
devices are TTY-compatible.
17