What type of device is used by the deaf or hearing impaired to communicate via telephone lines?

The TTY, or Text Telephone, is specifically designed for individuals who are deaf or hard of hearing, allowing them to communicate over standard telephone lines. Users type their messages on a keyboard, and the text is transmitted over the phone line to another TTY device, where it can be read on a screen. This technology provides a critical means of communication that does not rely on audio, making it accessible for those with hearing impairments.

In contrast, while VoIP (Voice over Internet Protocol) technology allows for voice communication over the internet, it does not cater specifically to the needs of the deaf community. P-ANI relates to automatic number identification in telecommunications but is not a communication device itself. Telematics usually involves telecommunications and monitoring systems, which also do not focus on facilitating communication for individuals who are deaf or hard of hearing. Thus, the TTY remains the appropriate device for the purpose described in the question.