You can use the iPhone TTY Adapter cable (sold separately in many areas) to connect iPhone to a TTY machine. Go to www.apple.com/store (may not be available in all areas) or check with your local Apple retailer.
Connect iPhone to a TTY machine. Go to Settings > Phone and turn TTY on, and then connect iPhone to your TTY machine using the iPhone TTY Adapter.
When TTY on iPhone is turned on, the TTY icon appears in the status bar at the top of the screen. For information about using a particular TTY machine, see the documentation that came with the machine.