High Quality Content by WIKIPEDIA articles! The Precise Tone Plan specification for the North American network defines the Call-progress tones used for standard signaling in the PSTN. Prior to the Precise Tone Plan, devices used other frequencies and levels. Such tones generated by older telephone exchange equipment did not follow the Precise Tone Plan and were either formed as dual frequency tones or equivalently as ring modulated signals where a higher frequency is modulated by a lower frequency. In telephony, call progress tones are audible tones sent from the public switched telephone network (PSTN) central office or a private branch exchange (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. ToneScript is a tone description format that may be used to specify the tone.

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