Precise tone plan
The precise tone plan is a signaling specification for the public switched telephone network 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 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:
- Dial tone is a continuous tone of the addition of the frequencies 350 and 440 Hz at a level of −13 dBm.
- Audible ringing 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.
- Low tone, also busy tone, is defined as having frequency components of 480 and 620 Hz at a level of −24 dBm and a cadence of one half second ON and one half second OFF. Reorder tone, also often called fast busy tone, is the same tone, 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 this signal, 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.
- High tone is a tone of 480 Hz at –17 dB.