Like a lot of things in life... it depends.
In short, screaming is a natural behaviour for birds, but we can make our birds' screaming better or worse based on our reaction to it.
The best approach for the long term is not rewarding screaming with attention (i.e. ignoring it).
Give your bird attention when it's behaving in a manner you'd like to continue (when it's quiet, or at least, calling in a way that's pleasant to you).
My GCC is currently 2 meters to my left, yelling out the window at the wild neighbourhood birds and sunshine. I'm ignoring her, because I know she's perfectly safe and not actually calling to me. I also know that particular screaming will pass when spring hormonal season does. If I really wanted her to stop, I'd do something on my own that I know she enjoys and she'd make the choice that she would rather do this other thing (go to the kitchen and make lunch, in our case).
One of my macaws has learned they need to call out with "hello!" if they want me to come get them (that's the behaviour I reinforced, but any positive behaviour is possible).
Ensuring they have appropriate enrichment to keep them entertained, and predictable routines will help matters too.
What's going on in their world during the, "ear piercing flock calling", and what's your reaction to them?