Teletype (TTY) machines are used by people who are deaf or hard of hearing to communicate by typing and reading text. If you have the iPhone TTY Adapter, available at www.apple.com/store, you can use iPhone with a TTY machine.
If you see in the status bar, it means that TTY is on. Choose Settings > Phone to turn TTY on or off.
TTY is only available if your carrier supports the feature. Contact your carrier for more information.
Learn more about the accessibilty options on your iPhone.