Abstract: Telephone call progress tones are identified by measuring the periods of their envelope cycles in the range of 20 to 200 Hertz which are due to the difference frequency of the signals used by phone company to form the call progress tones. The periods of the envelope cycles are classified, and a decision is made by a programmed microprocessor by comparing the data derived from the unknown signal to what is expected for known call progress calls of many telephone companies. In making such decision, the several period measurements are accumulated in bins, and the bin reaching the largest total is used to develop a representative period which is examined, if various other criteria are met, to identify the call. In this process, substantially regular envelope cycles are selected, and noise is rejected. An answer to the call is detected by this technique.