Wolf is correct, it won't matter where he is, if he doesn't see you or cannot get near you, he will scream. That, in a nutshell, is the problem everybody has with cockatoos and why they are given up all the time and end up going from one home to another: because normal people with normal lifestyles cannot keep them company 24/7/365 and that is the only thing that will satisfy them. Needing constant company is not their fault or even a luxury for them, it's an actual need because it's the way nature made them - and it's also why they don't make g.ood pets even though they are so very loving and funny.
I have two, both males and both came to me with what bird people call 'behavioral issues', Freddy was a screamer and Linus a plucker. It took ten months for Freddy to stop and I am still working on Linus although he is much, much better - but Freddy is what everybody wants in a cockatoo and only very few people get - he is funny, outgoing (a complete ham!), doesn't scream, eats great, loves his baths, doesn't pluck (well, just a little, he still plucks his legs every now and then). For ten months Freddy screamed all day long and, sometimes, even during the night. The only way he did not scream was when he was on me because, if he could see or hear me but he could not get to me, he would scream even if I was standing right next to his cage. I simply reassured him every time he screamed by walking over to his cage and talking to him while I touched him - just to have to do it all over again 5 minutes later
(it was a very trying time, especially with my husband complaining about the noise all the time and saying the neighbors were going to complain). Personally, I don't think that giving a reply flock call from another room works with cockatoos (it didn't work with Freddy) but try it and see what happens. Both my cockatoos now live in the birdroom and I am not there all the time but they live cage-free, have lots and lots of wood to chew, they have the company of other birds and they know I will be there for them in the am and the pm like clockwork because I am very strict when it comes to their schedule and routine so their days are always, 100% the same. Keeping a very strict schedule and routines helps them a lot with the anxiety that captivity brings to them (it's the anxiety caused by the fear/vulnerability that been alone that makes them scream or pluck) so I suggest you do it, too. Also, make sure he keeps to a strict solar schedule because cockatoos are considered 'hormonal birds' due to their having two breeding seasons a year and an overly-hormonal cockatoo is impossible to keep happy.