Telephone line signalling
Descriptive information about the various signals from/to the PSTN

Tones from PSTN Busy tone Call waiting tone Caller waiting tone Comfort tone Conference tone Confirmation tone Congestion tone Dial tone Disconnect tone End of three party service tone Executive override tone Holding tone Howler tone Intrusion tone Line lock-out tone Negative indication tone Notify tone Number unobtainable tone Off-hook warning tone Pay tone Payphone recognition tone Preemption tone Queue tone Recall dial tone Record tone Ringing tone Search tone Second dial tone Special dial tone Special information tone Waiting tone Warning tone

Signals to PSTN Answer tone Calling tone Guard tone Pulse (loop disconnect) dialling Tone (DTMF) dialling Other signals from PSTN Billing (metering) signal DC conditions Ringing signal Even more tones from PSTN >>> Show/hide Acceptance tone Audible ring tone Busy override warning tone Busy verification tone Engaged tone Facilities tone Fast busy tone Function acknowledge tone Identification tone Intercept tone Permanent signal tone Positive indication tone Re-order tone Refusal tone Ringback tone Route tone Service activated tone SIT Special ringing tone Stutter dial tone Switching tone Test number tone Test tone Trunk offering tone ITU-T annex to E.180 (obsolete) List of tones used in national networks Search World PSTN tone database