Abstract: A Call Filter System (CFS): (1) logically interfaces between a public telephone network and business customer premises equipment; (2) automatically processes incoming and outgoing calls which are amenable to such intervention; and (3) enlists the aid of human intervention on calls requiring such intervention.

Abstract: Apparatus for detecting a single-frequency signal having a predetermined frequency in an analog signal includes means for converting the analog signal to a differentially encoded digital signal and detection means for detecting a single-frequency signal having the predetermined frequency from the differentially encoded digital signal. In a preferred embodiment, the detection means includes: means for detecting one cycle of the analog signal by detecting a signal maximum followed by detecting a signal minimum which is followed, in turn, by detecting another signal maximum; means for determining the length of the cycle, for determining whether the length is within predetermined bounds, and for keeping a count of such occurrences; and means for determining whether the count has reached a predetermined amount, whereby a single-frequency signal having the predetermined frequency has been detected.