I had the same problem with my grey. Had him for twenty years. When I was on the phone, talking to someone or I left the room he would scream and shout. Did not stop until I acknowledged him.
I put it down to him being peed off because I was paying someone else more attention. In the early days I would then pet him and talk to him so that it would stop.
However, thinking about it, to him it was working so I started ignoring him instead. Took a few months but now he doesn't do it anymore.
All I can suggest is that you do the same and see what happens. Perseverance is the key.
Good luck.
Only tip I cathonling about it hn give you us do not shout back at him as this may make it worse.