Signal tone

Call progress tones (CPTone), also called signal tones, are generally composed of several discrete single-frequency tones that are played repeatedly on a make-break ratio basis. Signal tones, including dial tone, ringback tone, and busy tone, are used to inform users of the call progress. Signal tones vary greatly with countries as per their national standards.

Signal tones support two modes: country mode and custom mode. In addition, the amplitude of signal tones can be customized to avoid signal tone detection failure.