Patent ID: 7664243

Claim:
A system for detecting addition of a third party to a pre-existing telephonic connection between a first party and a second party, said system comprising: a meter for measuring an amplitude of a line voltage of the telephonic connection between the first party and the second party, said line voltage having a frequency band F 1 ; a first filter to remove a frequency band F 2 from said line voltage and output a filtered signal, wherein said frequency band F 2 is a sufficiently narrow subset of said frequency band F 1 that absence of said frequency band F 2 from said line voltage is substantially imperceptible to the first party and the second party; a random noise generator for generating a random noise signal having a frequency band corresponding to said frequency band F 2 and an amplitude that is sufficiently less than said amplitude of said line voltage so as to be substantially imperceptible to the first party and the second party when combined with said filtered signal; a coupler to combine said random noise signal with said filtered signal, thereby outputting an additive line voltage; a second filter that passes at least a portion of said additive line voltage, including at least a portion of said frequency band F 2 , said portion being referred to as a band-passed signal; and a processor for time-domain monitoring of an amplitude of said band-passed signal for a shift associated with addition of the third party to the pre-existing telephonic connection.