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:^[1]

• dial tone is a continuous tone having frequencies of 350 and 440 Hz at a level of −13 dBm
• ringback tone is defined as comprising frequencies of 440 and 480 Hz at a level of −19 dBm and a cadence of 2 seconds ON and 4 seconds OFF
• busy tone is defined as having frequency components of 480 and 620 Hz at a level of −24 dBm and a cadence of half a second ON and half a second OFF
• reorder tone, also called "fast busy" tone, contains the same frequency components as busy tone at a similar level but with a cadence of 0.25 of a second ON and 0.25 of a second OFF; the original plan had two slightly different versions of the "fast busy" tone, with a cadence of 0.2 of a second ON and 0.3 of a second OFF to signal toll-circuit congestion and a cadence of 0.3 of second ON and 0.2 of a second OFF for local reorder.

Prior to the Precise Tone Plan, parts of the Bell System and various switching systems used slightly varying signal frequencies and levels. The standardization process began with the installation of the first electronic switching system, a Western Electric 1ESS at Succasunna, NJ in 1965.^[2]

References

1. AT&T "Notes on Distance Dialing". 1968.
2. Bell Telephone Magazine, 41(2) p.63 (1968)