Voice Carry Over: An option of the relay service (See Relay) that allows a person (who can speak but not hear well), to talk on the phone with someone who can hear.
With a normal relay call, a relay operator, (called a "communication assistant" acting between the two communicating parties, will type what the hearing person says so that the other person can read it on their TTY or computer.
With a VCO relay call, the person who cannot hear does not have to type their message, but can speak it directly. VCO can make a relay call much more convenient for hard of hearing people or late deafened, since only one side of the conversation has to be typed and that is done by the relay operator.
A normal relay telephone call only requires a single phone line to the relay operator, but a VCO call requires two phone lines and is known as two-line VCO.
You can also do VCO calls using IP-Relay and that doesn't require a second phone line if your computer doesn't tie up your phone line when connecting to the Internet.