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.
Equipment is required to maintain tolerances within ± 0.5% in frequency and ±1.5 dB in amplitude stability.
The standardization process began with the installation of the first electronic switching system, a Western Electric 1ESS at Succasunna, NJ in 1965.
[3] All subsequent switching systems, such the 2/2B ESS, 4ESS, 5ESS, DMS-10, DMS-100, TOPS, EWSD, and NEAX-61E followed this practice.