The CMX673 is a low voltage multi-purpose Call Progress Tone detector for use in Public Switched Telephone Network System (PSTN) applications. Call progress detection allows equipment that dials into the PSTN to monitor the progress of the resulting call. Various states such as Ringing, Busy, Not available, and Answer can be determined. Using digital signal processing techniques the CMX673 accurately analyzes tones in the 315Hz to 650Hz frequency band and distinguishes valid call progress tone signals from line noise or voice. This contrasts with other call progress detection devices that are based on simple filtering techniques. When combined with cadence measurement of the CMX673 output, the rapid detect response time of the CMX673 allows it to support a wide range of call progress functions including `stutter dial tone' detection used in voice messaging systems. A single, low cost 3.58MHz crystal ensures accurate and repeatable performance. With supply requirements between 2.7V and 5.5V the CMX673 is easily integrated into a wide range of telecom applications. The convenient CMX673 pin out allows it to be readily incorporated into existing product designs to quickly achieve its benefits. Small geometry CMOS design techniques provide a complete call progress detector that analyzes both frequency and amplitude in a small single package. Available packages are: 8-pin PDIP (CMX673P1), 16-pin SOIC (CMX673D4), and 20-pin TSSOP (CMX673E3).