Abstract: To detect tones for call progress monitoring, hysteresis apparatus is provided whereby the output of a typical tone detector such as a comparator operating in conjunction with a selective filter is monitored by a counter. The counter is incremented and decremented according to whether the input is indicative of the presence or absence of a tone. If the counter reaches a predetermined count within a given period of time, then an indication is made that the tone exists. If the counter does not reach this predetermined count or threshold within the applicable time, then it is an indication that the tone does not exist. The thresholds of the counters plus the amount by which a counter may be incremented or decremented for each input signal is variable for each tone.