I could also be answering a phone call. I think browsing your phone is mostly considered rude by people, but answering a call would be more grey area. Depending on the nature of the meet up and the perceived urgency of the call (which would be hard to estimate for the rest of the people present).
At the core of etiquette lies consideration for others. Looking at your phone constantly while in a meeting tells everyone else that you think the meeting wastes your time, or that you can't pay attention. If you don't want to participate don't go to the meeting. If you must take calls during a meeting (with the ringer off I would hope) you excuse yourself so you don't disrupt the meeting and everyone else.
You don't have to know anyone's specific boundaries to show consideration. You treat people they way you would like them to treat you. You wouldn't appreciate people browsing their phones while you presented at a meeting. You wouldn't appreciate phones ringing and people taking calls while you spoke or wanted to listen. Basic Golden Rule stuff.
