TTY Relay Service is a communication service that helps people who are deaf, hard of hearing, or speech-impaired to communicate over the phone. It uses a device called a TTY (Text Telephone) that allows users to type messages, which are then relayed by a communication assistant to the hearing person on the other end of the call. The service works by connecting the TTY user to a relay operator, who reads the typed messages aloud to the hearing person and types their spoken responses back to the TTY user. This enables effective communication between individuals who use different methods of speaking, ensuring accessibility for all.