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.

The tones are as follows:

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.

...
Wikipedia