TTY mode on a cellphone activates the ability to use a cellphone as a teletypewriter, which is a text-like system used primarily by deaf people to communicate over the phone. To use TTY mode, the cellphone must be connected to a portable TTY device.
Both the sender and the recipient must use a TTY device, according to AboutTTY.com. According to Radio Shack, the main benefit of using a TTY system on a cellphone instead of simply sending text messages is that the TTY device offers an experience that is more like a conversation. Messages typed into the TTY device appear instantly on the other person's screen.