| 摘要 |
The apparatus of the invention (a) monitors a subscriber line for the presence of DTMF tones, (b) intercepts DTMF tones by transmitting a set of DTMF harmonics onto the subscriber line, (c) detects the intercepted DTMF tones and records them or the digits they represent, (d) analyzes the intercepted digits to determine whether a long distance number has been dialed, and (e) if a long distance call has been dialed, transmits onto the subscriber line a DTMF signal which includes the intercepted DTMF signal plus a service provider access code DTMF signal inserted before it. According to a presently preferred embodiment, the apparatus includes a voltage sensor which detects a drop in voltage whenever a phone connected to the same subscriber line as the apparatus goes off-hook. The voltage detector turns the apparatus "ON" when an off-hook condition is detected. The preferred apparatus also includes an energy sensor which rapidly detects the presence of any fixed frequency tone and triggers a harmonics tone generator which preferably generates second harmonics of each of the "row frequencies". Thus, all DTMF tones are blocked before they are recognized.
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