It's normal for cockatiels to be very vocal when they wake up in the morning and at night right before they go to sleep, this is their way to say "good morning" and "good night" to everybody. Tiels will also be very vocal if you have been out for a while and you come home to them, this is their way to greet you, and let you know that they are very happy you are home to play with them.
Now cockatiels can get very spoiled, and they can learn very fast how to control you. This is where excessive screaming might take place. If you come to the cage every time your cockatiel screams, your cockatiel will understand that screaming equals to you coming over. You do this a few times and your cockatiel will develop the ability to use their screams as a 'remote control' to make you come over.
• Do not feed your cockatiel in response to the screaming, if you do, you are just rewarding your tiel for what is doing.
• If you have been out of the house for a while, do spend sometime with your cockatiel.
• Moving your cockatiel to a new area of the house might increase screaming.
• Understand that cockatiels are not supposed to be quiet animals, so allow for normal noisy periods.
• Do not reward excessive screaming with attention and drama.
• Teach your cockatiel a contradictory sound, such as whistling or whispering.
• Always reinforce good noises with lavish praise and attention.
• Always ignore the sounds that you don't like, such as excessive screaming.
• Look for patterns in screaming episodes, then look for ways to prevent the pattern from starting.
It's difficult to make a cockatiel NOT to be loud. Difficult, but not impossible. It just take a lot of patience and time. Like most animals, cockatiels are conditioned to repetitive routines and tasks.
A human baby can learn very quickly that by crying, he can make mom come to his crib anytime he wants. If Mom comes every time the baby cries, the baby has trained mom to come at his will. Cockatiels are not different. A cockatiel knows that he can train you to come to the room if he squawks enough. You would have to introduce your bird to a routine where he knows you won't come when he squawks all the time. Tiels can be persistent and they will try to wear out your patience by just being louder than before. The important thing for you is not to pay attention, go for a walk watch TV but do not surrender to your tiels demands. Again, if your tiel has been doing this for a while it will take time to condition your cockatiel.