A graphical user interface for a web telephone and other telephony devices provides a unique combination of display elements that provide information and enable the user to access functionality of the device. The display elements include customizable screen areas called panes, an application program selection area, and call state area for displaying telephone line status information. A software platform of the user interface provides a set of default user interface features and exposes an application programming interface. System implementers may customize the default display elements or create entirely new custom panes that fit within a uniform user interface framework using the application programming interface.