@Vmpslushie this is less phone addiction and more "afraid of being left out" isn't it? When she first used the phone, she didn't talk to her "friends" for 4 hours which made them restless and talk badly about her. Idk why but girls' friendship can be very strict, if you fall behind you may be bullied. Or at least that's what I gathered from the manga I read.
Anw black girl was having toxic "friends" so she had to constantly be with them, because the moment she stopped doing so, they will abandon her. This made her ignore her only true friend, the blonde girl, and blondie was frustrated so she pulled a prank which unfortunately killed black girl's life. Frankly speaking, everyone was in the wrong, especially those "friends"