What is wrong with Holding on to their cell phone. It is pretty normal in a world where we havent created actual tribal structures with the people we vibe with, to at least be in contact all day like That.
Or it is coping mechanism like a distraction in Order to avoid feeling. Either way, I would focus on genuine connection instead of the behavior. If you dont get through in terms of communication to even get a sense of connection, connect on an emotional an energetical Level Even more like feel into their being. And than Mayen do nice thinks like little gestures like a cup of Hot chocolate while they are on the phone, so they see that they are Not judged and your willingness to connect is not conditional.
When my daughter spends a lot of time on the tablet and doesnt naturally come to me for connection I come to her every hour Or so und cuddle her and just flood her with love and appreciation. Anyway she is still smaller, so this might not be the appropriate way of connecting for everyone.