What device is designed specifically for deaf or hearing impaired individuals to communicate via telephone lines?

The device designed specifically for deaf or hearing-impaired individuals to communicate via telephone lines is the TTY, which stands for Text Telephone or Teletypewriter. TTY allows individuals to type messages back and forth over the phone line, enabling effective communication without the need for spoken words.

TTY devices convert typed messages into signals that can be transmitted over standard telephone lines, and they can also convert incoming messages back into text for the user to read. This technology has been vital in improving accessibility for people with hearing impairments, allowing them to engage in real-time conversations with both other TTY users and hearing individuals via relay services.

While other options like VoIP adapters and relay service devices also play roles in communication, they do not specifically cater to the needs of deaf or hearing-impaired individuals in the same direct way that TTY devices do. Digital messaging devices may offer electronic communication methods but lack the specific functionality tied to telephone lines that TTY provides.