There's a UNICOM voice channel?

No, unless in (I think) South Africa.

On IVAO, voice system is based on TeamSpeak. When you disconnect from an active channel, I believe the system will put you in UNICOM channel of selected server.Note that this is not a valid communication option, for several reasons, including that it does not reach text only pilots (which ATC will do himself) and is based on server, therefore will not reach valid pilots as text will (depending on position, altitude and I think speed)