Hearing Carry-Over (HCO)
A less common call type is Hearing Carry Over (HCO). The HCO method is used by people who have difficulty speaking, but have no difficulty hearing voice. HCO allows a speech-impaired person to type messages on a TTY (which are voiced by the relay operator) and then pick up the handset and listen to the other person's response. A variation of HCO is 2-line HCO. Similar to 2-Line Voice Carry Over (VCO), a speech-impaired person using a TTY to call a TRS operator, who in turn calls the user on a second telephone line, which serves as the voice line. The user puts the TRS operator on a brief hold to initiate a 3-way call with the Hearing party.
Online relay services (All Out of Date)
Relay Service for US Federal Agencies