View bu8874 datasheet:

Communication ICs DTMF receiver for telephones BU8874 / BU8874F The BU8874 and BU8874F are DTMF receiver ICs developed for use in telephone answering machines, and convert 16 different types of DTMF signals into 4-bit binary serial data. In addition to a compact 8-pin DIP (BU8874) package, these receivers feature a wide dynamic range, eliminating the need for an external input amplifier. Expertise from a num- ber of companies has been incorporated into these products to enable guard time control through a host microcontroller. FApplications Telephone answering machines FFeatures 1) Dynamic range of 45dB. (internal AGC) 6) 4.19MHz crystal resonator can be used. 2) Power down mode. 7) 8-pin DIP package. (BU8874) 3) 4-bit binary serial data output. 4) Guard time can be controlled