No, it's called TTY. And it's not text to speech. Think about it - deaf people can read text and can not hear speech. How would a deaf person benefit from text to speech?
Teletype (TTY) machines are used by deaf and hard of hearing people to communicate by typing and reading text. If you have the iPhone TTY Adapter cable, available at www.apple.com/store, you can use iPhone with a TTY machine. Choose Settings > Phone to turn TTY on or off.