Apparatus for converting a standard telephone to an unlisted number telephone

Claim 1

Apparatus for converting a standard telephone system including a conventional telephone instrument at a transmitting station (TS) which is conventionally connected by wire conduit to a central station (CS); and a receiving station (RS) having a conventional telephone instrument connected by conventional wire line to said central station; the improved apparatus adapted to be attached to said RS which would convert it to an unlisted number telephone, comprising: (a) switch means adapted to switch resistive load means across said line at the RS; (b) first validity test means capacitance coupled to the line at said RS to recognize the wave shape of a conventional ringing signal, and to determine its validity; (c) means responsive to said first validity test means, to produce for a selected time interval a selected first tone signal and apply it to said line; whereby responsive to receiving this first tone signal at said TS, the TS, via a DTMF system, transmits to said RS a private code which may be one or more decimal digits in a selected successive order; whereby said dialed numbers will produce a pattern of tones received at said RS; (d) capacitive coupled to said line at said RS in an analog to digital converter means (A/DC) and digital to decimal converter means (D/DC) for producing a series of decimal digit signals which pass to a second validity check means, to check that the dialed digits are valid; (e) a plurality of settable switches are connected serially to the output of said A/DC and D/DC; (f) third validity test means to determine that each successive decimal digit corresponds completely to the numbers set into said successive swtiches; and means responsive to each successive validation of a switch to set a corresponding latch; and (g) responsive to said set latches, logic means to enable one of a selected number of tone generators to apply their signals to a sound source; whereby the operator of said RS can determine from the particular tone or tones, which one of a selective group of possible callers, the present call is from, and whether or not he wants to speak with said caller.