A coin operated telephone station is provided with apparatus for detectinghen a called party has answered a call (initiated at the coin operated telephone station) by the measurement of incoming signal energy on the telephone line. The apparatus includes hardware and software that cooperate in providing efficient and reliable detection of called party answer for the purpose of accurately charging telephone users. A microprocessor samples the output from each of three specialized energy detectors every 20 milliseconds to determine whether called party answer has occurred. The energy detectors include: (1) call progress tones in the frequency band 200-700 Hz approximately; (2) special information tones; and (3) broadband energy in the frequency band above 200 Hz. Additionlly, the time duration of the call progress tones and the quiet intervals between them are measured and used in the answer detection process. Signal energy from the calling party in the 200-700 Hz frequency band is filtered from the transmitting path by a 1200 Hz high-pass filter until called party answer occurs. Such filtering precludes a user from interfering with the detection process while permitting a limited amount of information to be tranmitted in the event that the answer detector is slow to operate.
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