A VCO (Voice Carry Over) phone is a special phone with a text readout that allows you to use VCO mode when making a relay call.

A VCO phone supports two-line VCO mode and requires two separate phone lines for that mode.

A VCO phone also includes special features for hard of hearing people, including, a powerful volume boost, frequency control, a flashing light to indicate when the phone is ringing, a volume control on the ringer, a 3.5mm jack to plug in a neckloop or ALD and of course, it's HAC.

To see the latest on VCO phones, check out this Google Search.