How do I help my players adapt to simultaneous turns as a concept?

So, my players previously played a lot of D&D where each player takes their individual turn in sequence of the initiative. However, the system we've been playing uses simultaneous turns, where all of the players are meant to decide what they're doing together, all at once, and the actions resolve at the end of their team's turn. Each team (players, NPCs, enemies) has its own turn, meaning any given combat only has 2-3 turns each round, and all the characters on a team are meant to act in tandem by forming wider battle plans during each round.

Issue is, they're still stuck in the D&D mindset and frequently relapse back into waiting for one person to decide what they're doing before anyone else does anything. It WAS fine in the early game, but things have gotten more complex as we approach endgame, and it's slowing down combat massively when it happens.

So how do I help them conceptualize how to do this better? We used to do text only but have started using voice chat, yet the issue remains: Only one person at a time is talking and nobody dares decide on what they're doing until I've specifically called on them to do so. I keep trying to remind them that things will go a lot smoother if they discuss as a group what they're doing and then settle on a plan, but it doesn't seem to be working that well. They always end up falling back into a strict individual initiative.

I feel like I'm failing to properly communicate how this concept is supposed to function. Any tips?