Voice Carry-Over (VCO)

The next most common call type is Voice Carry Over (VCO). VCO allows the deaf or hard of hearing person to use his or her voice while receiving responses from a hearing person via text typed by the relay operator (also known as communication assistant or relay agent). There are many variations of VCO, including 2-Line VCO. 2-Line VCO allows a Hard of Hearing person using a TTY or computer 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. This method is frequently used by the Hard-of-Hearing who like to use some of their residual hearing as well not having to say "Go Ahead".

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

Online relay services (All Out of Date)

AT&T Relay
Hamilton Relay
i711.com Relay
Verizon Relay
Nordia Relay
Sorenson Relay
Sprint Relay
Relay Service for US Federal Agencies