In some regions, Teletype (TTY) machines are used by deaf or hearing-impaired people to communicate by typing and reading text. You can use iPhone with a TTY machine if you have the iPhone TTY Adapter cable, available for purchase separately in many countries. Go to www.apple.com/store (not available in all countries).
Choose Phone, then turn TTY on. Then connect iPhone to your TTY machine using the iPhone TTY Adapter.
For information about using a TTY machine, see the documentation that came with the machine.
For more information about other accessibility features of iPhone, go to Accessibility.