A technique for inhibiting intelligible interception of information signals transmitted over a line from a first site to a second site. A masking signal is applied to the line immediately upon the detection of the information signal at the second site, but prior to the time at which the validity of the...http://www.google.com/patents/US5168519?utm_source=gb-gplus-sharePatent US5168519 - System and method for securing DTMF transmission
