A switch issues different tones, to indicate the current activity on the phone line. However, any one activity, for example, ringing, is indicated differently by each switch; the tones may be switch-specific or defined by national standards. In addition, some switches send different tones depending on whether the target number is internal or external to the switch.
For example, when a ROLM switch receives a signal that the telephone receiver is off-hook, it generates a dial tone for a set number of seconds while waiting for dialing to begin, followed by a beep tone if the telephone is not dialed, followed by a howl tone after a set period of no activity.
For more information about all progress tones, see the Blueworx Voice Response for AIX: Configuring the System information.