TTY-based Telecommunications Relay Services permit persons with a hearing or speech disability to use the telephone system via a text telephone (TTY) or other device to call persons with or without such disabilities.

What happens when you call a TTY number?

A TTY is a special device that lets people who are deaf, hard of hearing, or speech-impaired use the telephone to communicate, by allowing them to type messages back and forth to one another instead of talking and listening. A TTY is required at both ends of the conversation in order to communicate.

What is TTY call mode?

Android offers support for TTY mode, which can mean "teletypewriter" or "text telephone" among other things. TTY mode is a communication tool allowing text communication over standard phone line connections as it converts the text input to audio and then decodes that audio back into text for reception.