Precise tone plan

The precise tone plan is a signaling specification for the public switched telephone network (PSTN) in North America. It defines the call-progress tones used for indicating the status and progress of telephone calls to subscribers and operators.
All signals in the specification use combination (by addition) of audible tones of four frequencies: 350 Hz, 440 Hz, 480 Hz, and 620 Hz. Equipment is required to maintain tolerances within ± 0.5% in frequency and ±1.5 dB in amplitude stability. Harmonic distortion is to be at least 30 dB below the applied tone level.
The tones are as follows:
Prior to the precise tone plan, parts of the Bell System and various switching systems used various similar signal frequencies and levels, without standardization, often referred to as "nonprecise" call progress tones. The standardization process began with the installation of the first electronic switching system, a Western Electric 1ESS at Succasunna, NJ in 1965. All subsequent switching systems, such the 2/2B ESS, 4ESS, 5ESS, DMS-10, DMS-100, TOPS, EWSD, and NEAX-61E followed this practice.