The status of a line device can change for many reasons, some as a result of requests submitted by the local application, and some as a result of actions performed by the switch or by the application (or person) at the other end of the connection.
In either case, an application is notified about these changes with the LINE_LINEDEVSTATE message, which indicates the status item (the attribute of the line device) that has changed. The application can choose the line status items for which it wants to be notified using the lineSetStatusMessages function. The messages controlled by invoking lineSetStatusMessages are LINE_LINEDEVSTATE and LINE_ADDRESSSTATE.
In addition, an application can determine the current status of an address by calling lineGetAddressStatus, which returns its information in a structure of the type LINEADDRESSSTATUS. It can also see the complete status of the specified open line device by calling lineGetLineDevStatus, which returns its information in a structure of the type LINEDEVSTATUS.