A monitor refers to a capability that watches for activity on a call or a device. A monitor placed on a device or a call causes reports of changes in the status of the device or call to be sent to the client requesting the monitor. If your application places a device monitor on your phone, your application is notified of changes in your phone’s status (for example, an incoming call has been received, a call ended, and so forth). Many applications rely on monitors to provide this type of information.