Mobile phones have become ubiquitous in many countries today. As the number of subscribers and geographical coverage continues to increase worldwide, mobile users can now be reached at nearly any place and at any time. Though the technology supports near perpetual connectivity, users are not available for mobile phone communication in such a straightforward way. As a result, users are often interrupted by mobile phones in the middle of activities. One reason that has been attributed to this problem is the callers lack of awareness about the callee’s current activity. Some context aware mobile technologies have been developed with an aim to help users coordinate mobile phone interruptions. However, these studies are mostly technology oriented and we are yet to understand the effectiveness of these designs. This is because there are few studies on interruption and availability in relation to mobile phone use. To examine the role of technology in coordinating mobile phone interruptions, we have conducted a diary study to investigate the factors that affect the disruptiveness of a mobile phone call, and the practices by which mobile users currently manage their availability. The results from our study show that the availability of a user is highly subjective, and that the interruption rating of a call is affected by factors such as location, activity, relationship with the caller as well as the history of interactions between them. We have also found that mobile phone users appropriate features of the device by way of silencing calls, dismissing calls or placing a ’missed call’, in order to manage their availability. The implications of our results for the design of technologies such as context aware telephony, that aim to coordinate mobile phone calls is discussed.