"I need it for the thing we're talking about"

Sometimes the phone is part of the conversation. They mention a place and you look it up. They ask what time the train is and you check. You're showing them the photo you were talking about. That isn't a glance and this book isn't against it.

The difference is easy to feel and easy to state. When the phone is part of the conversation, the other person can see the screen, or could, and knows why you're on it. When it's a glance, the screen is yours, the reason is yours, and they're waiting. Same object, different act. In the first, the phone is on the table between you. In the second it's a door you've stepped through.

Two things go wrong at the edge. The first is the errand that turns into a glance. You go to check the train time, and there's a notification, and you're reading it, and the train time was found twenty seconds ago. The person watching sees the moment your purpose ended and the drift began, and to them it looks exactly like the glance it's become. The second is the reverse. You tell yourself you're looking something up for the conversation and you're not, and they can tell, because nothing comes back from the screen.

The habit to build is a short sentence. "Hang on, let me find it." "I'll check what time it is." Out loud, before the phone comes up, and then the phone goes away when the thing is found. Said out loud, the errand can't turn into a glance without you hearing yourself do it. And the other person is with you on the screen instead of on the far side of it.

So the phone can be in the conversation. It just can't be in it silently, and it has to leave when its job is done.