ITU standard for Multifrequency. Very detailed and good for your study on dtmf

A Low-Complexity ITU-Compliant DTMF decoder (Evans algorithm).

双音多频 DTMF（Dual Tone Multi Frequency），双音多频，由高频群和低频群组成，高低频群各包含4个频率。-Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communic

This class provides methods to play DTMF tones (ITU-T Recommendation Q.23), call supervisory tones (3GPP TS 22.001, CEPT) and proprietary tones (3GPP TS 31.111). Depending on call state and routing options, tones are mixed to the downlink audio or ou