Signaling system 7 is an international system used to communicate between telephone offices and other telecom systems. It conveys information about the status of each end of the call to the other. For example, it detects when the remote instrument is being rung and signals the originating office to play a "ringing" signal to the originating unit. It is also used for informing each end that the call has been answered and that it has been disconnected (later), which information is often critical for proper billing and for freeing up circuits that are no longer being used.