Call of Duty Warzone Crossplay Voice Chat: How to Enable
By Jack Cohen

Call of Duty Warzone has had crossplay capabilities since launch, but the issue then becomes how does communication work? Thankfully, Activision has thought about that and the normal party chat can be extended to include crossplay users.
Call of Duty Warzone Crossplay Voice Chat: How to Enable
To start, with the Call of Duty: Warzone home screen displayed, press the Options key on your controller. Slide all the way over to the account tab and scroll down and the second option should read "Crossplay Communications."
Make sure the option is enabled and you should then be able to communicate with your crossplay squad-mates. Just make sure that all members of your squad also have crossplay communication enabled, or else it won't work. You may need to restart your game for this to work fully. See below for a visual guide.
Make sure you show Activision how important this feature is by using it often, so they make sure to incorporate it in any future games they create.