Most Deaf people do not use this anymore because it is ancient technology. Deaf people use cell phones to text and Video Phones at home to communicate with others. The TTY should be replaces everywhere with VP's and other Deaf friendly devices. Would you want to type everything you were thinking on a type writer at this day in age???
This is probably NOT what you are asking, but ...
capitolized like that means "teletype"
It's how deaf people handle regular telephone calls. The phone call goes thru a special operator who changes it from spoken word to printed word on a teletype machine, and back again.
TTY stands for Text Telephone. It is also sometimes called a TDD, or Telecommunication Device for the Deaf.
do you mean ttyl cause that means " talk to you later "
It's probably meant to be Talk To You Later. (ttyl)