Automated prefix dialing system

Abstract

An automated prefix dialing system is disclosed to allow a subscriber/user to automatically insert a predetermined prefix dialing code in certain outgoing telephone call situations. The system is electrically disposed between the telephone company's central office circuit and the subscriber's circuit. The system preferably comprises six sub-circuits: DC power path circuit (off-hook sensor circuit), shadow-ring detector circuit, isolation circuit, DTMF receiver circuit, DTMF generator circuit, and micro controller circuit. When not in use, no power is used by the system. The DC power path circuit, provides power received from the central office line to the various circuits and components of the system when the subscriber's phone is taken off-hook. Off-hook sensor circuitry detects when the telephone has been lifted either in response to an incoming call or to initiate an outgoing call. The shadow ring circuit informs the system if the phone is taken off hook because of an incoming call and functions without any power. When an outgoing call is made, the isolation circuit isolates, the subscriber's phone from the central office line to allow the system to analyze the number being dialed by the user. If the number satisfies certain calling conditions the programmed prefix digits are automatically inserted before the number by the system. Whether or not prefix digits are inserted, DTMF tones, representing the number dialed by the user, must be re-generated by the system and are ultimately transmitted to the central office line, with or without the prefix digits, by the isolation circuit when isolation is removed.