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.

Source: http://en.wikipedia.org/wiki/Telecommunications_Relay_Service#Hearing_Carry_Over

Online relay services (All Out of Date)

AT&T Relay
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