Precise Tone Plan

The Precise Tone Plan specification for the North American network defines the Call-progress tones used for standard signaling in the PSTN. The tones are as follows:

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.

References