Why can't these people just get up and LEAVE THE ROOM, as a courtesy to the other PAYING GUESTS?
Doing that would cause a certain amount of disruption. If someone needs to actually speak on the phone, the person should get up and leave. On the other hand, if a phone allowed a person to listen to the caller on an earpiece and give a few simple responses to questions by silently pushing buttons without removing his phone from his pocket, a person who had to be on call for certain situations might be able to address them with less disruption than would result from the person's leaving the room. An anticipated scenario would be when a person attending some computer equipment observes an out-of-the-ordinary condition, which can "probably" be solved a certain way, while the person who is in charge of the equipment is at a concert or other such event. The person attending the equipment could describe the symptoms and the expected fix (or possibly a small number of possible fixes), and the person in charge could indicate that he should (1) go ahead with a fix, (2) hang up and expect a phone call very shortly; or (3) hang up, expect to be called back after some length of time. In short, the idea would be to avoid having the person leave the room (which would cause some disruption for the other concert-goers) for calls which weren't important enough to justify it.