This is why the typical broader view implementation used a bandpass filter to detect presence of frequencies in the general range of the tones and then uses firmware to detect the cadence of the tone. The cadence is as much an indication of the CP state as the tone frequency itself.

A call progress mode, using the MT8880C, can be selected allowing the detection of various tones which identify the progress of a telephone call on the network. The call progress tone input and DTMF input are common, however, call progress tones can only be detected when CP mode has been selected. DTMF signals cannot be detected if CP mode has been selected (see Table 5). Figure 8 indicates the useful detect bandwidth of the call progress filter. Frequencies presented to the input, which are within the ‘accept’ bandwidth limits of the filter, are hard-limited by a high gain comparator with the IRQ/CP pin serving as the output. The squarewave output obtained from the schmitt trigger can be analyzed by a microprocessor or counter arrangement to determine the nature of the call progress tone being detected. Frequencies which are in the ‘reject’ area will not be detected and consequently the IRQ/CP pin will remain low.