Interaction shield, or involvement barrier, is a term first used by sociologist Erving Goffman to describe social cues that are turned on through active or inactive shifts in verbal or non-verbal cues.
The social cues of a cell phone user denote that the person is "engaged" in a phone call and not engaged in the nearby social space around them. He is in two places at once, but because of his social cue of holding the phone to his ear, the other "free social radicals" in the nearby social space understand that they may not interact with him.
In Behavior in Public Places, Goffman notes that people may "expect to find a variety of barriers to perception used as involvement shields, behind which individuals can safely do the kind of things that ordinarily result in negative sanctions"  In the case of a phone call, the negative sanction would be a user talking to oneself in public. Because he has established an interaction shield, he is free to talk as he pleases. In most cases, an interaction shield involves blocking perception of either bodily signs of involvment or objects of involement or both. In the case of a cell phone user, part of the conversation is removed from view. More courteous cell phone users may engage in phone calls while turning their body or head away from a social gathering in order to achieve some privacy while still showing their engagement with an external actor.