Just join a squad and hit "B" (default setting for VOIP in squads) and talk to your squad mates...see if they respond. Make sure you're on a server with VOIP inabled. In the server selection screen, to the left of the server name there will be an icon of headphones if the server has VOIP. Sometimes your squad mates will talk to you, sometimes not. I usually play with friends so we always squad up and us it. Helps a crap load when your in an attack chopper using the voip with your co-pilot.