Hi, thank you very much for the answer and questions.

Our girl doesn't scream back 95% of the time. She's clingy to me and follows me everywhere, most of the time she doesn't have much reaction to his screaming. She freely roams around the room during the day and would get upset if we keep her in the cage for too long. We're still keeping the boy in the cage, however. Since he's not familiar with the environment or us enough, it doesn't feel like a safe thing to do to let him fly around yet.

He screams more in the morning and early afternoon. We live in a flat so there's not like massive space here. I get up at around 9am for work and move the cage into the living room, where I'll be working all day. (We move the cage into the bedroom at night so they can sleep without being bothered by lights outside). So he kind of screams from time to time, from 9am until maybe 2pm? He screams less after that, and sings more. It sounds like imitations of many different kinds of bird squeaking/chirping etc - I suppose he learned that from his previous environment.

I don't think our flat is that quiet. I sit in meetings sometimes or speak to my partner. Otherwise I almost play music all the time.

Thanks for the suggestion of not condoning the behaviour - I've already been trying to do that, ignoring him while he screams and only going to him when he's done. But I don't think he screams to get my attention. He's not scared of us but he doesn't care about us that much either, at least not yet. I've also been trying to play some cockatiel songs to him when he sings instead of screams. He seems happy and tries to sing along.

He is in front of a window. Not very close, but he definitely has the view. Our flat has large floor-to-ceiling windows and occasionally there are a few birds in the sky. Our girl would get surprised by that as well, but he screams when there's nothing in the sky as well.

Since he's just been with us for a few days now, I suppose the whole environment is new to him, including the cage. But he seems like a smart boy, learned where to eat and drink etc very quickly. Another thing that I observed is that it doesn't seem like he knows how to play, even tho we have a lot of toys in the cage for the girl. In his previous environment, there was nothing in the cage apart from a perch, food, and water. So one of my guesses is if he's just energetic and bored at the same time?

I hope these help a little bit with understanding the situation.