In this paper we describe the results of a study investigating the behaviour and views of bystanders in response to a proximal mobile telephone conversation by a third party. Analysis of the data revealed that despite varied expressed views on embarrassment, discomfort and rudeness, patterns of behaviour were remarkably similar. Mechanisms of disengagement were employed by all of the participants so that they were demonstrably not attending; yet all of them were able to report on the precise content of the overheard calls. Other social mechanisms were used by the bystanders to diffuse the perceived intrusiveness of the call and to grant "permissions" for these intrusions. Implications are drawn from the study for the design of mobile and ubiquitous computing applications.