Telecommunications relay service or sometimes also called a dual-party telephone relay service. It enables text telephone users to communicate with a non-text telephone user by way of a relay service communications assistant. The ADA mandated this nationwide relay service by 1993. There are several available types of this service: voice carry over, VCO with privacy, two line VCO, hearing carry over, HCO with privacy, two line HTO, DeafBlind variation, speech to speech, captioned telephone etc.
Voice carry over allows a person with hearing loss to receive a text of what the other person have said. Hearing carry over is not as common type. It allows allows a person who is speech-disabled but can hear to use their hearing while sending responses to a person who is hearing via the HCO user’s typed text. Speech to speech is for people with speech disabilities. A TRS operator who is specially trained revoices what the person with the disability wanted to say. DefBlind variation exist for people who are both def and blind. Captioned telephone (also called captioned relay or Cap-Tel) is a hybrid communication method that enables people who are hard of hearing, oral deaf or late–deafened to make phone calls. It is a telephone that displays real-time captions of the current conversation.