If you are constantly chekcing your teens phone they will start to grow distant from you because you are invading their privacy and they sometimes want to have secrets and they will tell you if they want to. They will start to get angrier and more frustrated with you and will get agitated when you ask questions like "What are you doing?" or "Give me your phone."
It can cause a form of distrust between the parent and child. Unless the child did something to deserve that. Usually when this happens, a child has either been lying, hiding, or did something they aren't suppose to. This has happened to me before and its becasue ym mother is a very strict person. This can from a bad bond between the parent. Also it causes them to be so much more rebelious. You want to control that, so it's best to sit down and talk to them.