The Precise Tone Plan specification for the North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

n network defines the Call-progress tone

Call-progress tone

In telephony, call progress tones are audible tones sent from the public switched telephone network central office or a private branch exchange to calling parties to indicate the status of phone calls....

s used for standard signaling in the PSTN. The tones are as follows:

• dial tone
  Dial tone
  A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...
  
   is a continuous tone having frequencies of 350 and 440 Hz
  Hertz
  The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
  
   at a level of −13 dBm
  DBm
  dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
  
  
• ringback tone
  Ringback tone
  A ringback tone is an audible indication that is heard on the telephone line by the caller while the phone they are calling is being rung. It is normally a repeated tone, designed to assure the calling party that the called party's line is ringing, although the ring-back tone may be out of sync...
  
   is defined as comprising frequencies of 440 and 480 Hz
  Hertz
  The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
  
   at a level of −19 dBm
  DBm
  dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
  
   and a cadence of 2 seconds ON and 4 seconds OFF
• busy tone is defined as having frequency components of 480 and 620 Hz
  Hertz
  The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
  
   at a level of −24 dBm
  DBm
  dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
  
   and a cadence of half a second ON and half a second OFF
• reorder tone
  Reorder tone
  The reorder tone, also known as the fast busy tone, is the congestion tone or all trunks busy tone of a public switched telephone network. It varies from country to country; in the USA it is a dual-frequency tone of 480 Hz and 620 Hz at a cadence of 0.25 seconds on, 0.25 off; that is two beeps per...
  
  , 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

Prior to the Precise Tone Plan, devices used other frequencies and levels. Such tones generated by older telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 equipment did not follow the Precise Tone Plan and were either formed as dual frequency tones or equivalently as ring modulated

Ring modulation

Ring modulation is a signal-processing effect in electronics, an implementation of amplitude modulation or frequency mixing, performed by multiplying two signals, where one is typically a sine-wave or another simple waveform. It is referred to as "ring" modulation because the analog circuit of...

signals where a higher frequency is modulated by a lower frequency.