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In telephony, call progress tones are audible tones sent from the PSTN or a PBX to calling parties to indicate the status of phone calls.
Equipment such as fax machines and modems are designed to recognise dial tones and busy tones.
The ITU-T E.180 and E.182 recommendations define the technical characteristics and intended usage of some of these tones.
Types
Major ones encountered by the public on a regular basis are listed in bold:
- busy tone or busy signal
- call waiting tone
- comfort tone
- conference call tone
- confirmation tone
- congestion tone
- dial tone
- end of three-party service tone (three-way calling)
- executive override tone
- holding tone
- howler tone
- intercept tone
- intrusion tone
- line lock-out tone
- negative indication tone
- notify tone
- number unobtainable tone
- pay tone
- payphone recognition tone
- permanent signal tone
- preemption tone
- queue tone
- recall dial tone
- record tone
- ringback tone or ringing tone
- ringtone or ringing signal
- second dial tone
- special dial tone
- special information tone (SIT)
- waiting tone
- warning tone
