A user interface for screen phones monitors telephone line state information and displays user interface controls and telephone line status information dependent on the telephone line state. The user interface includes a display element referred to as a call slip that expands and collapses in response to changes in telephone line state and user input. The call slip updates the information that it displays based on telephone line state changes and information communicated from one or more telephony monitoring applications. The call slip displays user interface controls representing tasks that are dependent on information derived from a telephone call, such as caller ID information. The software used to control the display of call slips and to monitor telephone line state is part of a software platform that may be customized for a particular device or application. For example, an implementer may select default display elements and monitoring application programs from the platform and add custom display elements that communicate with the default programs via their programming interfaces.