Switch TTY/TDD mode on to make calls with a TTY/TDD device. These devices are for the speech or hearing impaired.
- On the start screen, swipe left, and tap Settings.
- Tap ease of access.
- Under TTY/TDD mode, tap Full.
The available modes may vary depending on your network service provider.
For more info on using your phone with a TTY/TDD, go to http://www.microsoft.com/windowsphone/en-gb/howto/wp7/basics/use-my-phone-with-a-tty.aspx.